cifs: Change key name to cifs.idmap, misc. clean-up

Change idmap key name from cifs.cifs_idmap to cifs.idmap.
Removed unused structure wksidarr and function match_sid().
Handle errors correctly in function init_cifs().

Signed-off-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>