[SCSI] sd: revive sd_index_lock

Commit f27bac27 which converted sd to
use ida instead of idr incorrectly removed sd_index_lock around id
allocation and free.  idr/ida do have internal locks but they protect
their free object lists not the allocation itself.  The caller is
responsible for that.  This missing synchronization led to the same id
being assigned to multiple devices leading to oops.

Reported and tracked down by Stuart Hayes of Dell.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Stable Tree <stable@kernel.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>