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

Given a partial block at the end of a payload, aes_encrypt_icm() would
perform a 16-byte load of the residual into a temporary variable. This
is unsafe in principle since the full block may cross a page boundary.
Fix the problem by copying the residual into a stack buffer first.

Reported by: syzbot+b7e44cde9e2e89f0f6c9@syzkaller.appspotmail.com
Reported by: syzbot+4b5eaf123a99456b5160@syzkaller.appspotmail.com
Reported by: syzbot+70c74c1aa232633355ca@syzkaller.appspotmail.com
Reported by: syzbot+2c663776a52828373d41@syzkaller.appspotmail.com
Reviewed by: cem, jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31730