xfs: allow empty transactions while frozen

xfs: allow empty transactions while frozen

In commit fe5a728e3a24347 ("xfs: implement the GETFSMAP ioctl") we
created the ability to obtain empty transactions.  These transactions
have no log or block reservations and therefore can't modify anything.
Since they're also NO_WRITECOUNT they can run while the fs is frozen,
so we don't need to WARN_ON about that usage.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>