The "blocked_list" used for POSIX deadlock detection should only contain
POSIX locks, or posix_locks_deadlock() can get confused.

As I've noted before (but never posted a patch), we should probably stop
checking for deadlock.  POSIX says it's optional for an implementation
to check for deadlock, and since the change to match on tgid, we can
report a deadlock when none exists.

[ Further explanation for Linus: POSIX locks and BSD locks don't block
  each other, so checking that the blocker is a POSIX lock also implies
  that the waiter is a POSIX lock. ]