The performance using aes-gcm for SMB3 encryption is very poor compared to
aes-ccm. (Using our software implenentation is both cases).

Callgrind showed that we use 28,165,720,719 cpu cycles to deliver
a 100MB file to a client using aes-ccm.

With aes-gcm this is raises up to 723,094,413,831 cpu cycles.

As a reference copying without signing or encryption uses
just 134,264,902 cpu cycles. And signing with aes-cmac uses
13,321,036,015 cycles. While SMB2 signing with sha256 uses
14,124,659,113 cycles.

We should optimize the aes-{cmac,ccm,gcm} software implementions.
The most problematic parts are the xor and right- and leftshift
implementations.

As it seems that software aes-gcm will still be much slower than
software aes-ccm, we should disable aes-gcm support for 4.3.0.