GFS2: Don't try to end a non-existent transaction in unlink

Before this patch, if function gfs2_unlink failed to get a valid
transaction (for example, not enough journal blocks) it would go
to label out_end_trans which did gfs2_trans_end. But if the
trans_begin failed, there's no transaction to end, and trying to
do so results in: kernel BUG at fs/gfs2/trans.c:117!

This patch changes the goto so that it does not try to end a
non-existent transaction.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>