block: fix loop on blk_drain_queue when __dm_destroy

block: fix loop on blk_drain_queue when __dm_destroy

when install apk to sdcard, vold will loop on blk_drain_queue:
vold            D c090e3a8     0   119      1 0x00000000
[<c090e3a8>] (__schedule+0x3b0/0x85c) from [<c090ed98>] (schedule_timeout+0x160/0x2dc)
[<c090ed98>] (schedule_timeout+0x160/0x2dc) from [<c047de38>] (msleep+0x14/0x20)
[<c047de38>] (msleep+0x14/0x20) from [<c05e6e04>] (blk_cleanup_queue+0xb4/0x218)
[<c05e6e04>] (blk_cleanup_queue+0xb4/0x218) from [<c071cca4>] (__dm_destroy+0x170/0x1c4)
[<c071cca4>] (__dm_destroy+0x170/0x1c4) from [<c0721d8c>] (dev_remove+0x74/0xa8)
[<c0721d8c>] (dev_remove+0x74/0xa8) from [<c0722534>] (dm_ctl_ioctl+0x1e4/0x2d4)
[<c0722534>] (dm_ctl_ioctl+0x1e4/0x2d4) from [<c05153c4>] (do_vfs_ioctl+0x80/0x5b0)
[<c05153c4>] (do_vfs_ioctl+0x80/0x5b0) from [<c051592c>] (sys_ioctl+0x38/0x60)
[<c051592c>] (sys_ioctl+0x38/0x60) from [<c0439540>] (ret_fast_syscall+0x0/0x30)