crypto: sha1-mb - make sha1_x8_avx2() conform to C function ABI

Megha Dey reported a kernel panic in crypto code.  The problem is that
sha1_x8_avx2() clobbers registers r12-r15 without saving and restoring
them.

Before commit aec4d0e3 ("x86/asm/crypto: Simplify stack usage in
sha-mb functions"), those registers were saved and restored by the
callers of the function.  I removed them with that commit because I
didn't realize sha1_x8_avx2() clobbered them.

Fix the potential undefined behavior associated with clobbering the
registers and make the behavior less surprising by changing the
registers to be callee saved/restored to conform with the C function
call ABI.

Also, rdx (aka RSP_SAVE) doesn't need to be saved: I verified that none
of the callers rely on it being saved, and it's not a callee-saved
register in the C ABI.

Fixes: aec4d0e3 ("x86/asm/crypto: Simplify stack usage in sha-mb functions")
Cc: stable@vger.kernel.org # 4.6
Reported-by: Megha Dey <megha.dey@linux.intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>