xfs: alert the user about data/attr fork mappings that could be merged

If the data or attr forks have mappings that could be merged, let the
user know that the structure could be optimized.  This isn't a
filesystem corruption since the regular filesystem does not try to be
smart about merging bmbt records.

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