gfs2: Clean up gfs2_unstuff_dinode

Split __gfs2_unstuff_inode off from gfs2_unstuff_dinode and clean up the
code a little.  All remaining callers now pass NULL as the page argument
of gfs2_unstuff_dinode, so remove that argument.

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