[XFS] Delay direct I/O completion to a workqueue This is nessecary

because aio+dio completions may happen from irq context but we need
process context for converting unwritten extents.  We also queue regular
direct I/O completions to workqueue for regularity, there's only one
queue_work call per syscall.

SGI-PV: 934766
SGI-Modid: xfs-linux:xfs-kern:196857a

Signed-off-by: Christoph Hellwig <hch@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>