librbd: prepare lock_acquire() for changing between policies

librbd: prepare lock_acquire() for changing between policies

In preparation for adding a new TransientPolicy, get rid of the check
implemented in terms of exclusive_lock::Policy::may_auto_request_lock()
that essentially makes it so that exclusive lock policy on a given
image handle can be changed from the default AutomaticPolicy only once.
In order to effect another change a new image handle would have been
needed which is pretty suboptimal.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>