lockd: Pass "struct sockaddr *" to new failover-by-IP function

Pass a more generic socket address type to nlmsvc_unlock_all_by_ip() to
allow for future support of IPv6.  Also provide additional sanity
checking in failover_unlock_ip() when constructing the server's IP
address.

As an added bonus, provide clean kerneldoc comments on related NLM
interfaces which were recently added.

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