xfs: PF_FSTRANS should never be set in ->writepage

Now that we reject direct reclaim in addition to always using GFP_NOFS
allocation there's no chance we'll ever end up in ->writepage with
PF_FSTRANS set.  Add a WARN_ON if we hit this case, and stop checking
if we'd actually need to start a transaction.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>