xfs: move xfs_bmap_finish prototype

This function is used libxfs code, but is implemented separately in
userspace. Move the function prototype to xfs_bmap.h so that the
prototype is shared even if the implementations aren't.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>