fs/super.c: use && instead of & for warn_on condition

This fixes the following sparse warning:

  fs/super.c:1202:9: warning: dubious: x & !y

Bitwise and logical and are equivalent here, but logical was intended.
The generated code is identical, with and without CONFIG_LOCKDEP.

Signed-off-by: Vincent Stehlé <vincent.stehle@freescale.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>