xfs: don't print warnings when xfs_log_force fails

There are only two reasons for xfs_log_force / xfs_log_force_lsn to fail:
one is an I/O error, for which xlog_bdstrat already logs a warning, and
the second is an already shutdown log due to a previous I/O errors.  In
the latter case we'll already have a previous indication for the actual
error, but the large stream of misleading warnings from xfs_log_force
will probably scroll it out of the message buffer.

Simply removing the warnings thus makes the XFS log reporting significantly
better.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>