xfs: give the deferred barrier op type a name

xfs_barrier_defer_type is the only xfs_defer_op_type with no .name.
Every other one carries a short string used for tracing and reporting:
attr, bmap, extent_free, agfl_free, rtextent_free, refcount,
rtrefcount, rmap, rtrmap and exchmaps.

That has been harmless because nothing dereferences the field, but it
leaves a NULL in a table where every other entry is populated, so the
first caller to print it gets "(null)" in the kernel and undefined
behaviour in the userspace libxfs build of this file, where xfs_alert
lands in fprintf.  xfs_defer_add() already treats a missing member of
this table as worth shutting the filesystem down for, so an unpopulated
one is out of step with how the file handles its own ops tables.

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

@@ -229,6 +229,7 @@ xfs_defer_barrier_cancel_item(
 }
 
 static const struct xfs_defer_op_type xfs_barrier_defer_type = {
+	.name		= "barrier",
 	.max_items	= 1,
 	.create_intent	= xfs_defer_barrier_create_intent,
 	.abort_intent	= xfs_defer_barrier_abort_intent,