locks: fix possible infinite loop in posix deadlock detection

It's currently possible to send posix_locks_deadlock() into an infinite
loop (under the BKL).

For now, fix this just by bailing out after a few iterations.  We may
want to fix this in a way that better clarifies the semantics of
deadlock detection.  But that will take more time, and this minimal fix
is probably adequate for any realistic scenario, and is simple enough to
be appropriate for applying to stable kernels now.

Thanks to George Davis for reporting the problem.

Cc: "George G. Davis" <gdavis@mvista.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>