xfs: document another racy GC case in xfs_zoned_map_extent

xfs: document another racy GC case in xfs_zoned_map_extent

Besides blocks being invalidated, there is another case when the original
mapping could have changed between querying the rmap for GC and calling
xfs_zoned_map_extent.  Document it there as it took us quite some time
to figure out what is going on while developing the multiple-GC
protection fix.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hans Holmberg <hans.holmberg@wdc.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>