crypto: arm/aes-neonbs - work around gcc-15 warning

I get a very rare -Wstringop-overread warning with gcc-15 for one function
in aesbs_ctr_encrypt():

arch/arm/crypto/aes-neonbs-glue.c: In function 'ctr_encrypt':
arch/arm/crypto/aes-neonbs-glue.c:212:1446: error: '__builtin_memcpy' offset [17, 2147483647] is out of the bounds [0, 16] of object 'buf' with type 'u8[16]' {aka 'unsigned char[16]'} [-Werror=array-bounds=]
  212 |                         src = dst = memcpy(buf + sizeof(buf) - bytes,
arch/arm/crypto/aes-neonbs-glue.c: In function 'ctr_encrypt':
arch/arm/crypto/aes-neonbs-glue.c:218:17: error: 'aesbs_ctr_encrypt' reading 1 byte from a region of size 0 [-Werror=stringop-overread]
  218 |                 aesbs_ctr_encrypt(dst, src, ctx->rk, ctx->rounds, bytes, walk.iv);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
arch/arm/crypto/aes-neonbs-glue.c:218:17: note: referencing argument 2 of type 'const u8[0]' {aka 'const unsigned char[]'}
arch/arm/crypto/aes-neonbs-glue.c:218:17: note: referencing argument 3 of type 'const u8[0]' {aka 'const unsigned char[]'}
arch/arm/crypto/aes-neonbs-glue.c:218:17: note: referencing argument 6 of type 'u8[0]' {aka 'unsigned char[]'}
arch/arm/crypto/aes-neonbs-glue.c:36:17: note: in a call to function 'aesbs_ctr_encrypt'
   36 | asmlinkage void aesbs_ctr_encrypt(u8 out[], u8 const in[], u8 const rk[],

This could happen in theory if walk.nbytes is larger than INT_MAX and gets
converted to a negative local variable.

Keep the type unsigned like the orignal nbytes to be sure there is no
integer overflow.

Fixes: c8bf850e ("crypto: arm/aes-neonbs-ctr - deal with non-multiples of AES block size")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>