[XFS] Warn if errors come from block_truncate_page().

block_truncate_page() can return errors that we currently ignore and
silently discard. We should not ever get errors reported here - an error
indicates a bug somewhere else. Hence catch the error and issue a stack
dump to the syslog because we cannot propagate the error any further up
the call chain.

SGI-PV: 980084
SGI-Modid: xfs-linux-melb:xfs-kern:30800a

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