xen/blkfront: fix warning when deleting gendisk on unplug/shutdown

Currently blkfront gives a warning when hot unplugging due to calling
del_gendisk() with interrupts disabled (due to blkif_io_lock).

WARNING: at kernel/softirq.c:124 local_bh_enable+0x36/0x84()
Modules linked in: xenfs xen_netfront ext3 jbd mbcache xen_blkfront
Pid: 13, comm: xenwatch Not tainted 2.6.29-xs5.5.0.13 #3
Call Trace:
 [<c012611c>] warn_slowpath+0x80/0xb6
 [<c0104cf1>] xen_sched_clock+0x16/0x63
 [<c0104710>] xen_force_evtchn_callback+0xc/0x10
 [<c0104e32>] check_events+0x8/0xe
 [<c0104d9b>] xen_restore_fl_direct_end+0x0/0x1
 [<c0103749>] xen_mc_flush+0x10a/0x13f
 [<c0105bd2>] __switch_to+0x114/0x14e
 [<c011d92b>] dequeue_task+0x62/0x70
 [<c0123b6f>] finish_task_switch+0x2b/0x84
 [<c0299877>] schedule+0x66d/0x6e7
 [<c0104710>] xen_force_evtchn_callback+0xc/0x10
 [<c0104710>] xen_force_evtchn_callback+0xc/0x10
 [<c012a642>] local_bh_enable+0x36/0x84
 [<c022f9a7>] sk_filter+0x57/0x5c
 [<c0233dae>] netlink_broadcast+0x1d5/0x315
 [<c01c6371>] kobject_uevent_env+0x28d/0x331
 [<c01e7ead>] device_del+0x10f/0x120
 [<c01e7ec6>] device_unregister+0x8/0x10
 [<c015f86d>] bdi_unregister+0x2d/0x39
 [<c01bf6f4>] unlink_gendisk+0x23/0x3e
 [<c01ac946>] del_gendisk+0x7b/0xe7
 [<d0828c19>] blkfront_closing+0x28/0x6e [xen_blkfront]
 [<d082900c>] backend_changed+0x3ad/0x41d [xen_blkfront]

We can fix this by calling del_gendisk() later in blkfront_closing, after
releasing blkif_io_lock. Since the queue is stopped during the interrupts
disabled phase I don't think there is any danger of an event occuring between
releasing the blkif_io_lock and deleting the disk.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>