locks: don't call locks_release_private from locks_copy_lock

All callers of locks_copy_lock pass in a brand new file_lock struct, so
there's no need to call locks_release_private on it. Replace that with
a warning that fires in the event that we receive a target lock that
doesn't look like it's properly initialized.

Acked-by: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: Jeff Layton <jlayton@primarydata.com>