cifs: pull cifs_mount() call up

... to the point prior to sget().  Now we have cifs_sb set up early
enough.

Acked-by: Pavel Shilovsky <piastryyy@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>