gfs2: Lift withdraw check out of gfs2_ail1_empty

Lift the check for the SDF_WITHDRAWING flag out of gfs2_ail1_empty() and
into its callers.  This is needed so that gfs2_flush_revokes() can drop
the sd_log_lock spinlock before triggering a withdraw if necessary.

Instead of checking for the SDF_WITHDRAWING flag, use
gfs2_withdrawing().  Also, the low-level code triggering the delayed
withdraw reports when there is a problem, so there is no need to report
that again.

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