gfs2: minor evict fix

In evict_should_delete(), when gfs2_upgrade_iopen_glock() fails, we
detach the iopen glock from the inode without calling
glock_clear_object().  This leads to a warning in glock_set_object()
when the same inode is recreated and the glock is reused.
Fix that by only detaching the iopen glock in gfs2_evict_inode().

In addition, remove the dequeue code from evict_should_delete(); we
already perform a conditional dequeue in gfs2_evict_inode().

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