cifs: Call the calc_signature functions directly

As the SMB1 and SMB2/3 calc_signature functions are called from separate
sign and verify paths, just call them directly rather than using a function
pointer.  The SMB3 calc_signature then jumps to the SMB2 variant if
necessary.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Enzo Matsumiya <ematsumiya@suse.de>
cc: Paulo Alcantara <pc@manguebit.org>
cc: Shyam Prasad N <sprasad@microsoft.com>
cc: Tom Talpey <tom@talpey.com>
cc: linux-cifs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>