virtio: console: rename cvq_lock to c_ivq_lock

virtio: console: rename cvq_lock to c_ivq_lock

commit 165b1b8bbc17c9469b053bab78b11b7cbce6d161 upstream.

The cvq_lock was taken for the c_ivq.  Rename the lock to make that
obvious.

We'll also add a lock around the c_ovq in the next commit, so there's no
ambiguity.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Reviewed-by: Asias He <asias@redhat.com>
Reviewed-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
[bwh: Backported to 3.2:
 - Adjust context
 - Drop change to virtcons_restore()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>