[GFS2] Update init_dinode() to reduce stack usage

We no longer allocate a dinode on the stack in init_dinode()
and we no longer use gfs2_dinode_out (eliminating one copy) and
gfs2_meta_header_in (eliminating another copy). The meta_header_in
fucntion is now no longer referenced from outside gfs2_ondisk.c, so
make it static.

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