aesni: Avoid a potential out-of-bounds load in AES_GCM_encrypt()

[markj]

Summary

This is effectively the same problem as the one fixed in 564b6aa7fccd
("aesni: Avoid a potential out-of-bounds load in aes_encrypt_icm()")
but I didn't have a reproducer before.

Reported by: Jenkins

Test Plan

The ktls tests trigger this when KASAN is enabled. With this
change they pass. cryptocheck -d aesni0 -z -a aes-gcm{,192,256} also
runs successfully.

[cem]

sys/crypto/aesni/aesni_ghash.c

510

Can we not use _mm_xor_si128(tmp1, last_block)? Or we assume ^ generates the same instruction?

[markj]

sys/crypto/aesni/aesni_ghash.c

510

The aesni code does both but we should stay consistent within this function, I'll switch to the intrinsic.

[markj]

Use the xor intrinsic

[cem]

Cool