cifs: introduce cifs_dirent

Introduce a generic directory entry structure, and factor the parsing
of the various on the wire structures that can represent one into
a common helper.  Switch cifs_entry_is_dot over to use it as a start.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>