CIFS: Fix possible use after free in demultiplex thread

The recent changes that added SMB3 encryption support introduced
a possible use after free in the demultiplex thread. When we
process an encrypted packed we obtain a pointer to SMB session
but do not obtain a reference. This can possibly lead to a situation
when this session was freed before we copy a decryption key from
there. Fix this by obtaining a copy of the key rather than a pointer
to the session under a spinlock.

Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <smfrench@gmail.com>