locks: add lock cancel command

Lock managers need to be able to cancel pending lock requests.  In the case
where the exported filesystem manages its own locks, it's not sufficient just
to call posix_unblock_lock(); we need to let the filesystem know what's
happening too.

We do this by adding a new fcntl lock command: FL_CANCELLK.  Some day this
might also be made available to userspace applications that could benefit from
an asynchronous locking api.

Signed-off-by: Marc Eshel <eshel@almaden.ibm.com>
Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>