smb3.1.1: allow dumping GCM256 keys to improve debugging of encrypted shares

smb3.1.1: allow dumping GCM256 keys to improve debugging of encrypted shares

Previously we were only able to dump CCM or GCM-128 keys (see "smbinfo keys" e.g.)
to allow network debugging (e.g. wireshark) of mounts to SMB3.1.1 encrypted
shares.  But with the addition of GCM-256 support, we have to be able to dump
32 byte instead of 16 byte keys which requires adding an additional ioctl
for that.

Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>