[XFS] Fix a race in xfs_submit_ioend() where we can be completing I/O for

a page while we are still submitting other buffers on the same page for
I/O.

SGI-PV: 948197
SGI-Modid: xfs-linux-melb:xfs-kern:25004a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>