xfs: prohibit fstrim in norecovery mode

The xfs fstrim implementation uses the free space btrees to find free
space that can be discarded.  If we haven't recovered the log, the bnobt
will be stale and we absolutely *cannot* use stale metadata to zap the
underlying storage.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>