cifs: change confusing field serverName (to ip_addr)

ses->serverName is not the server name, but the string form
of the ip address of the server.  Change the name to ip_addr
to avoid confusion (and fix the array length to match
maximum length of ipv6 address).

Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>