Currently, gfs2_scan_glock_lru() decrements lru_count when a glock is
moved onto the dispose list.  When such a glock is then stolen from the
dispose list while gfs2_dispose_glock_lru() doesn't hold the lru_lock,
lru_count will be decremented again, so the counter will eventually go
negative.

This bug has existed in one form or another since at least commit
97cc1025 ("GFS2: Kill two daemons with one patch").

Fix this by only decrementing lru_count when we actually remove a glock
and schedule for it to be unlocked and dropped.  We also don't need to
remove and then re-add glocks when we can just as well move them back
onto the lru_list when necessary.

In addition, return the number of glocks freed as we should, not the
number of glocks moved onto the dispose list.
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>