GFS2: move function foreach_leaf to gfs2_dir_exhash_dealloc

The previous patches made function gfs2_dir_exhash_dealloc do nothing
but call function foreach_leaf.  This patch simplifies the code by
moving the entire function foreach_leaf into gfs2_dir_exhash_dealloc.

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