GFS2: Fix & clean up GFS2 rename

This patch fixes a locking issue in the rename code by ensuring that we hold
the per sb rename lock over both directory and "other" renames which involve
different parent directories.

At the same time, this moved the (only called from one place) function
gfs2_ok_to_move into the file that its called from, so we can mark it
static. This should make a code a bit easier to follow.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Cc: Peter Staubach <staubach@redhat.com>