dlm: improve plock logging if interrupted

This patch changes the log level if a plock is removed when interrupted
from debug to info. Additional it signals now that the plock entity was
removed to let the user know what's happening.

If on a dev_write() a pending plock cannot be find it will signal that
it might have been removed because wait interruption.

Before this patch there might be a "dev_write no op ..." info message
and the users can only guess that the plock was removed before because
the wait interruption. To be sure that is the case we log both messages
on the same log level.

Let both message be logged on info layer because it should not happened
a lot and if it happens it should be clear why the op was not found.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>