Commit 2e60d768 ("GFS2: update freeze code to use freeze/thaw_super
on all nodes") optimized away the sb_start_intwrite ... sb_end_intwrite
protection for the on-stack transactions in gfs2_ail_empty_gl with no
explanation.  I can't think of a valid reason for doing that, so revert
that change.  This simplifies the next commit.
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>