nfsd: use a long for the count in nfsd4_state_shrinker_count()

If there are no courtesy clients then the return value from the
atomic_long_read() could overflow an int. Use a long to store the value
instead.

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