[SCSI] bsg: use lib/idr.c to find a unique minor number

This replaces the current linear search for a unique minor number with
lib/idr.c.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>