nfsd: fix oops on mixed NFSv4/NFSv3 client access

If an NFSv2/v3 client breaks an NFSv4 client's delegation, it will hit a
NULL dereference in nfsd_breaker_owns_lease().

Easily reproduceable with for example

	mount -overs=4.2 server:/export /mnt/
	sleep 1h </mnt/file &
	mount -overs=3 server:/export /mnt2/
	touch /mnt2/file

Reported-by: Robert Dinse <nanook@eskimo.com>
Fixes: 28df3d15 ("nfsd: clients don't need to break their own delegations")
BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=208807


Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>