cifs: move allocation of new TCP_Server_Info into separate function

Clean up cifs_mount a bit by moving the code that creates new TCP
sessions into a separate function. Have that function search for an
existing socket and then create a new one if one isn't found.

Also reorganize the initializion of TCP_Server_Info a bit to prepare
for cleanup of the socket connection code.
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>