gfs2: Rename gfs2_withdrawn to gfs2_withdrawing_or_withdrawn

This function checks whether the filesystem has been been marked to be
withdrawn eventually or has been withdrawn already.  Rename this
function to avoid confusing code like checking for gfs2_withdrawing()
when gfs2_withdrawn() has already returned true.

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