gfs2: Turn gfs2_meta_indirect_buffer into gfs2_meta_buffer

Instead of only supporting GFS2_METATYPE_DI and GFS2_METATYPE_IN blocks,
make the block type a parameter of gfs2_meta_indirect_buffer and rename
the function to gfs2_meta_buffer.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>