[XFS] propogate return codes from flush routines

[XFS] propogate return codes from flush routines

This patch handles error return values in fs_flush_pages and
fs_flushinval_pages. It changes the prototype of fs_flushinval_pages so we
can propogate the errors and handle them at higher layers. I also modified
xfs_itruncate_start so that it could propogate the error further.

SGI-PV: 961990
SGI-Modid: xfs-linux-melb:xfs-kern:28231a

Signed-off-by: Lachlan McIlroy <[email protected]>
Signed-off-by: Stewart Smith <[email protected]>
Signed-off-by: Tim Shimmin <[email protected]>