cifs: add tcon field to cifsFileInfo struct

Eventually, we'll have more than one tcon per superblock. At that point,
we'll need to know which one is associated with a particular fid. For
now, this is just set from the cifs_sb->tcon pointer, but eventually
the caller of cifs_new_fileinfo will pass a tcon pointer in.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>