cifs: allocate mountdata earlier

pull mountdata allocation up, so that it won't stand in the way when
we lift cifs_mount() to location before sget().

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