GFS2: Prevent BUG from occurring when normal Withdraws occur

When the GFS2 file system withdraws due to metadata corruption, it
often has outstanding transactions in the journal and delayed work
queued for its glocks. This patch adds some new checks for a
withdrawn file system before proceeding with operations that would
obviously cause a BUG() to be triggered. That allows GFS2 to be
safely unmounted rather than cause the system to go down.

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