nfsd4: Expose the callback address and state of each NFS4 client

[ Upstream commit 3518c866 ]

In addition to the client's address, display the callback channel
state and address in the 'info' file.

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>