gfs2: Flush delete work before shrinking inode cache

In gfs2_kill_sb(), flush the delete work queue after setting the
SDF_DEACTIVATING flag.  This ensures that no new inodes will be
instantiated anymore, and the inode cache will be empty after the
following kill_block_super() -> generic_shutdown_super() ->
evict_inodes() call.

With that, function gfs2_make_fs_ro() now calls gfs2_flush_delete_work()
after the workqueue has been destroyed.  Skip that by checking for the
presence of the SDF_DEACTIVATING flag.

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