crypto: ansi_cprng - Avoid incorrect extra call to _get_more_prng_bytes

While working with some FIPS RNGVS test vectors yesterday, I discovered a
little bug in the way the ansi_cprng code works right now.

For example, the following test vector (complete with expected result)
from http://csrc.nist.gov/groups/STM/cavp/documents/rng/RNGVS.pdf

 ...

Key = f3b1666d13607242ed061cabb8d46202
DT = e6b3be782a23fa62d71d4afbb0e922fc
V = f0000000000000000000000000000000
R = 88dda456302423e5f69da57e7b95c73a

...when run through ansi_cprng, yields an incorrect R value
of e2afe0d794120103d6e86a2b503bdfaa.

If I load up ansi_cprng w/dbg=1 though, it was fairly obvious what was
going wrong:

----8<----
getting 16 random bytes for context ffff810033fb2b10
Calling _get_more_prng_bytes for context ffff810033fb2b10
Input DT: 00000000: e6 b3 be 78 2a 23 fa 62 d7 1d 4a fb b0 e9 22 fc 
Input I: 00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
Input V: 00000000: f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
tmp stage 0: 00000000: e6 b3 be 78 2a 23 fa 62 d7 1d 4a fb b0 e9 22 fc 
tmp stage 1: 00000000: f4 8e cb 25 94 3e 8c 31 d6 14 cd 8a 23 f1 3f 84 
tmp stage 2: 00000000: 8c 53 6f 73 a4 1a af d4 20 89 68 f4 58 64 f8 be 
Returning new block for context ffff810033fb2b10
Output DT: 00000000: e7 b3 be 78 2a 23 fa 62 d7 1d 4a fb b0 e9 22 fc 
Output I: 00000000: 04 8e cb 25 94 3e 8c 31 d6 14 cd 8a 23 f1 3f 84 
Output V: 00000000: 48 89 3b 71 bc e4 00 b6 5e 21 ba 37 8a 0a d5 70 
New Random Data: 00000000: 88 dd a4 56 30 24 23 e5 f6 9d a5 7e 7b 95 c7 3a 
Calling _get_more_prng_bytes for context ffff810033fb2b10
Input DT: 00000000: e7 b3 be 78 2a 23 fa 62 d7 1d 4a fb b0 e9 22 fc 
Input I: 00000000: 04 8e cb 25 94 3e 8c 31 d6 14 cd 8a 23 f1 3f 84 
Input V: 00000000: 48 89 3b 71 bc e4 00 b6 5e 21 ba 37 8a 0a d5 70 
tmp stage 0: 00000000: e7 b3 be 78 2a 23 fa 62 d7 1d 4a fb b0 e9 22 fc 
tmp stage 1: 00000000: 80 6b 3a 8c 23 ae 8f 53 be 71 4c 16 fc 13 b2 ea 
tmp stage 2: 00000000: 2a 4d e1 2a 0b 58 8e e6 36 b8 9c 0a 26 22 b8 30 
Returning new block for context ffff810033fb2b10
Output DT: 00000000: e8 b3 be 78 2a 23 fa 62 d7 1d 4a fb b0 e9 22 fc 
Output I: 00000000: c8 e2 01 fd 9f 4a 8f e5 e0 50 f6 21 76 19 67 9a 
Output V: 00000000: ba 98 e3 75 c0 1b 81 8d 03 d6 f8 e2 0c c6 54 4b 
New Random Data: 00000000: e2 af e0 d7 94 12 01 03 d6 e8 6a 2b 50 3b df aa 
returning 16 from get_prng_bytes in context ffff810033fb2b10
----8<----

The expected result is there, in the first "New Random Data", but we're
incorrectly making a second call to _get_more_prng_bytes, due to some checks
that are slightly off, which resulted in our original bytes never being
returned anywhere.

One approach to fixing this would be to alter some byte_count checks in
get_prng_bytes, but it would mean the last DEFAULT_BLK_SZ bytes would be
copied a byte at a time, rather than in a single memcpy, so a slightly more
involved, equally functional, and ultimately more efficient way of fixing this
was suggested to me by Neil, which I'm submitting here. All of the RNGVS ANSI
X9.31 AES128 VST test vectors I've passed through ansi_cprng are now returning
the expected results with this change.

Signed-off-by: Jarod Wilson <jarod@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>