[SCSI] bsg: fix bsg_mutex hang with device removal

We don't need to hold bsg_mutex during bsg_complete_all_commands(). It
leads to a problem that we block bsg_unregister_queue during
bsg_complete_all_commands (untill all the outstanding commands
complete).

Thanks to Pete Wyckoff for finding the bug and testing the patch.

The detailed bug report is:

http://marc.info/?l=linux-scsi&m=121182137132145&w=2

Tested-by: Pete Wyckoff <pw@osc.edu>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>