gfs2: Minor gfs2_try_evict cleanup

In gfs2_try_evict(), when an inode can't be evicted, we are grabbing a
temporary reference on the inode glock to poke that glock.  That should
be safe, but it's easier to just grab an inode reference as we already
do earlier in this function.

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