nfsd: use clear_and_wake_up_bit()

nfsd has two places that open-code clear_and_wake_up_bit().  One has
the required memory barriers.  The other does not.

Change both to use clear_and_wake_up_bit() so we have the barriers
without the noise.

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