xfs: drop freeze protection when running GETFSMAP

A recent log refactoring patchset from Brian Foster relaxed fsfreeze
behavior with regards to the buffer cache -- now freeze only waits for
pending buffer IO to finish, and does not try to drain the buffer cache
LRU.  As a result, fsfreeze should no longer stall indefinitely while
fsmap runs.  Drop the sb_start_write calls around fsmap invocations.

While we're cleaning things, add a comment to the xfs_trans_alloc_empty
call explaining why we're running around with empty transactions.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>