tcmu: remove commands_lock

No need for the commands_lock. The cmdr_lock is already held during
idr addition and deletion, so just grab it during traversal.

Note: This also fixes a issue where we should have been using at
least _bh locking in tcmu_handle_completions when taking the commands
lock to prevent the case where tcmu_handle_completions could be
interrupted by a timer softirq while the commands_lock is held.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>