GFS2: Flush the GFS2 delete workqueue before stopping the kernel threads

Flushing the workqueue can cause operations to happen which might
call gfs2_log_reserve(), or get stuck waiting for locks taken by such
operations.  gfs2_log_reserve() can io_schedule(). If this happens, it
will never wake because the only thing which can wake it is gfs2_logd()
which was already stopped.

This causes umount of a gfs2 filesystem to wedge permanently if, for
example, the umount immediately follows a large delete operation.

When this occured, the following stack trace was obtained from the
umount command

[<ffffffff81087968>] flush_workqueue+0x1c8/0x520
[<ffffffffa0666e29>] gfs2_make_fs_ro+0x69/0x160 [gfs2]
[<ffffffffa0667279>] gfs2_put_super+0xa9/0x1c0 [gfs2]
[<ffffffff811b7edf>] generic_shutdown_super+0x6f/0x100
[<ffffffff811b7ff7>] kill_block_super+0x27/0x70
[<ffffffffa0656a71>] gfs2_kill_sb+0x71/0x80 [gfs2]
[<ffffffff811b792b>] deactivate_locked_super+0x3b/0x70
[<ffffffff811b79b9>] deactivate_super+0x59/0x60
[<ffffffff811d2998>] cleanup_mnt+0x58/0x80
[<ffffffff811d2a12>] __cleanup_mnt+0x12/0x20
[<ffffffff8108c87d>] task_work_run+0x7d/0xa0
[<ffffffff8106d7d9>] exit_to_usermode_loop+0x73/0x98
[<ffffffff81003961>] syscall_return_slowpath+0x41/0x50
[<ffffffff815a594c>] int_ret_from_sys_call+0x25/0x8f
[<ffffffffffffffff>] 0xffffffffffffffff

Signed-off-by: Tim Smith <tim.smith@citrix.com>
Signed-off-by: Mark Syms <mark.syms@citrix.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>