rbd: lock_on_read map option

Add a per-device option to acquire exclusive lock on reads (in addition
to writes and discards).  The use case is iSCSI, where it will be used
to prevent execution of stale writes after the implicit failover.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Tested-by: Mike Christie <mchristi@redhat.com>