crypto: jitterentropy - build without sanitizer

Recent clang-9 snapshots double the kernel stack usage when building
this file with -O0 -fsanitize=kernel-hwaddress, compared to clang-8
and older snapshots, this changed between commits svn364966 and
svn366056:

crypto/jitterentropy.c:516:5: error: stack frame size of 2640 bytes in function 'jent_entropy_init' [-Werror,-Wframe-larger-than=]
int jent_entropy_init(void)
    ^
crypto/jitterentropy.c:185:14: error: stack frame size of 2224 bytes in function 'jent_lfsr_time' [-Werror,-Wframe-larger-than=]
static __u64 jent_lfsr_time(struct rand_data *ec, __u64 time, __u64 loop_cnt)
             ^

I prepared a reduced test case in case any clang developers want to
take a closer look, but from looking at the earlier output it seems
that even with clang-8, something was very wrong here.

Turn off any KASAN and UBSAN sanitizing for this file, as that likely
clashes with -O0 anyway.  Turning off just KASAN avoids the warning
already, but I suspect both of these have undesired side-effects
for jitterentropy.

Link: https://godbolt.org/z/fDcwZ5


Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>