xfs: initialise error in xfs_defer_finish_one()

xfs_defer_finish_one() declares error without an initialiser and only
assigns it inside the loop over dfp->dfp_work.  When that list is empty
the loop body never runs, control falls through to the "Done with the
dfp, free it" path, and the function returns an indeterminate value.

An item-less pending item reaches this through xfs_defer_add_barrier(),
which xfs_reap_ag_blocks() uses on any CONFIG_XFS_ONLINE_REPAIR kernel.
xfs_defer_finish_noroll() treats any non-EAGAIN return as fatal, so a
non-zero stack value turns a successful barrier into a
SHUTDOWN_CORRUPT_INCORE in the middle of a repair.  Zero is the correct
result: reaching the free path means the item loop drained without a
non-zero error.

Fixes: 3f3cec0310 ("xfs: force small EFIs for reaping btree extents")
Cc: stable@vger.kernel.org
Signed-off-by: Javier Tia <floss@jetm.me>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

fs/xfs/libxfs/xfs_defer.c

@@ -583,7 +583,7 @@ xfs_defer_finish_one(
 	const struct xfs_defer_op_type	*ops = dfp->dfp_ops;
 	struct xfs_btree_cur		*state = NULL;
 	struct list_head		*li, *n;
-	int				error;
+	int				error = 0;
 
 	trace_xfs_defer_pending_finish(tp->t_mountp, dfp);