crypto: skcipher - fix crash in virtual walk

The new skcipher walk API may crash in the following way. (Interestingly,
the tcrypt boot time tests seem unaffected, while an explicit test using
the module triggers it)

  Unable to handle kernel NULL pointer dereference at virtual address 00000000
  ...
  [<ffff000008431d84>] __memcpy+0x84/0x180
  [<ffff0000083ec0d0>] skcipher_walk_done+0x328/0x340
  [<ffff0000080c5c04>] ctr_encrypt+0x84/0x100
  [<ffff000008406d60>] simd_skcipher_encrypt+0x88/0x98
  [<ffff0000083fa05c>] crypto_rfc3686_crypt+0x8c/0x98
  [<ffff0000009b0900>] test_skcipher_speed+0x518/0x820 [tcrypt]
  [<ffff0000009b31c0>] do_test+0x1408/0x3b70 [tcrypt]
  [<ffff0000009bd050>] tcrypt_mod_init+0x50/0x1000 [tcrypt]
  [<ffff0000080838f4>] do_one_initcall+0x44/0x138
  [<ffff0000081aee60>] do_init_module+0x68/0x1e0
  [<ffff0000081524d0>] load_module+0x1fd0/0x2458
  [<ffff000008152c38>] SyS_finit_module+0xe0/0xf0
  [<ffff0000080836f0>] el0_svc_naked+0x24/0x28

This is due to the fact that skcipher_done_slow() may be entered with
walk->buffer unset. Since skcipher_walk_done() already deals with the
case where walk->buffer == walk->page, it appears to be the intention
that walk->buffer point to walk->page after skcipher_next_slow(), so
ensure that is the case.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>