xfs: fix debug_object WARN at xfs_alloc_vextent()

Fengguang reports:

[  780.529603] XFS (vdd): Ending clean mount
[  781.454590] ODEBUG: object is on stack, but not annotated
[  781.455433] ------------[ cut here ]------------
[  781.455433] WARNING: at /c/kernel-tests/sound/lib/debugobjects.c:301 __debug_object_init+0x173/0x1f1()
[  781.455433] Hardware name: Bochs
[  781.455433] Modules linked in:
[  781.455433] Pid: 26910, comm: kworker/0:2 Not tainted 3.4.0+ #51
[  781.455433] Call Trace:
[  781.455433]  [<ffffffff8106bc84>] warn_slowpath_common+0x83/0x9b
[  781.455433]  [<ffffffff8106bcb6>] warn_slowpath_null+0x1a/0x1c
[  781.455433]  [<ffffffff814919a5>] __debug_object_init+0x173/0x1f1
[  781.455433]  [<ffffffff81491c65>] debug_object_init+0x14/0x16
[  781.455433]  [<ffffffff8108842a>] __init_work+0x20/0x22
[  781.455433]  [<ffffffff8134ea56>] xfs_alloc_vextent+0x6c/0xd5

Use INIT_WORK_ONSTACK in xfs_alloc_vextent instead of INIT_WORK.
Reported-by: NWu Fengguang <wfg@linux.intel.com>
Signed-off-by: NJie Liu <jeff.liu@oracle.com>
Signed-off-by: NBen Myers <bpm@sgi.com>

fs/xfs/xfs_alloc.c

@@ -2441,7 +2441,7 @@ xfs_alloc_vextent(
 	DECLARE_COMPLETION_ONSTACK(done);
 
 	args->done = &done;
-	INIT_WORK(&args->work, xfs_alloc_vextent_worker);
+	INIT_WORK_ONSTACK(&args->work, xfs_alloc_vextent_worker);
 	queue_work(xfs_alloc_wq, &args->work);
 	wait_for_completion(&done);
 	return args->result;