NFS: Increase size of cl_ipaddr field to hold IPv6 addresses

The nfs_client's cl_ipaddr field needs to be larger to hold strings that
represent IPv6 addresses.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: Aurelien Charbon <aurelien.charbon@ext.bull.net>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>