[GFS2] Tidy gfs2_unstuffer_page

Tidy up gfs2_unstuffer_page by:

 a) Moving it into bmap.c
 b) Making it static
 c) Calling it directly from gfs2_unstuff_dinode
 d) Updating all callers of gfs2_unstuff_dinode due to one less
    required argument.

It doesn't change the behaviour at all.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>