cifs: have cifsFileInfo hold an extra inode reference

It's possible that this struct will outlive the filp to which it is
attached. If it does and it needs to do some work on the inode, then
it'll need a reference.

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