[XFS] Catch unwritten extent conversion errors.

On unwritten I/O completion, we fail to propagate an error when converting
the extent to a written extent. This means that the I/O silently fails.
propagate the error onto the ioend so that the inode is marked with an
error appropriately.

SGI-PV: 980084
SGI-Modid: xfs-linux-melb:xfs-kern:30826a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>