nfsd: use nfsd_v4client() in nfsd_breaker_owns_lease()

nfsd_breaker_owns_lease() currently open-codes the same test that
nfsd_v4client() performs.

With this patch we use nfsd_v4client() instead.

Also as i_am_nfsd() is only used in combination with kthread_data(),
replace it with nfsd_current_rqst() which combines the two and returns a
valid svc_rqst, or NULL.

The test for NULL is moved into nfsd_v4client() for code clarity.

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