librbd: Allow get_lock_info to fail

If the lock class isn't present, EOPNOTSUPP is returned for lock calls
on newer OSDs, but sadly EIO on older; we need to treat both as
acceptable failures for RBD images.  rados lock list will still fail.

Fixes #3744.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 4483285c9fb16f09986e2e48b855cd3db869e33c)