crypto: x86/aes-xts - optimize size of instructions operating on lengths

x86_64 has the "interesting" property that the instruction size is
generally a bit shorter for instructions that operate on the 32-bit (or
less) part of registers, or registers that are in the original set of 8.

This patch adjusts the AES-XTS code to take advantage of that property
by changing the LEN parameter from size_t to unsigned int (which is all
that's needed and is what the non-AVX implementation uses) and using the
%eax register for KEYLEN.

This decreases the size of aes-xts-avx-x86_64.o by 1.2%.

Note that changing the kmovq to kmovd was going to be needed anyway to
make the AVX10/256 code really work on CPUs that don't support 512-bit
vectors (since the AVX10 spec says that 64-bit opmask instructions will
only be supported on processors that support 512-bit vectors).

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>