gfs2: Clean up gfs2_scan_glock_lru

Switch to list_for_each_entry_safe() and eliminate the "skipped" list in
gfs2_scan_glock_lru().

At the same time, scan the requested number of items to scan, not one
more than that number.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>