cifs: define server-level cache index objects and register them

Define server-level cache index objects (as managed by TCP_ServerInfo structs)
and register then with FS-Cache. Each server object is created in the CIFS
top-level index object and is itself an index into which superblock-level
objects are inserted.

The server objects are now keyed by {IPaddress,family,port} tuple.

Signed-off-by: Suresh Jayaraman <sjayaraman@suse.de>
Signed-off-by: Steve French <sfrench@us.ibm.com>