lockd: remove SIGKILL handling

lockd allows SIGKILL and responds by dropping all locks and restarting
the grace period.  This functionality has been present since 2.1.32 when
lockd was added to Linux.

This functionality is undocumented and most likely added as a useful
debug aid.  When there is a need to drop locks, the better approach is
to use /proc/fs/nfsd/unlock_*.

This patch removes SIGKILL handling as part of preparation for removing
all signal handling from sunrpc service threads.

Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>