[XFS] Sanitise xfs_log_force error checking.

xfs_log_force() is declared to return an error, but we almost never check
it. We don't need to check it in most cases; if there's a log I/O error
then we'll be shutting down the filesystem anyway and that means we'll
catch the error somewhere else.

However, on certain calls we should be returning an error - sync
transactions, fsync, sync writes, etc. so this isn't a pure black and
white distinction. Hence make xfs_log_force() a void function that issues
a warning to the syslog on error, and call _xfs_log_force() in all the
places where we actually care about the error status returned.

SGI-PV: 980084
SGI-Modid: xfs-linux-melb:xfs-kern:30832a

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