Rename the current gfs2_iomap_get and gfs2_iomap_alloc functions to __*.
Add a new gfs2_iomap_get helper that doesn't expose struct metapath.
Rename gfs2_iomap_get_alloc to gfs2_iomap_alloc.  Use the new helpers
where they make sense.
Signed-off-by: NAndreas Gruenbacher <agruenba@redhat.com>