selinux: fix error codes in cond_read_av_list()

After this patch cond_read_av_list() no longer returns -1 for any
errors.  It just propagates error code back from lower levels.  Those can
either be -EINVAL or -ENOMEM.

I also modified cond_insertf() since cond_read_av_list() passes that as a
function pointer to avtab_read_item().  It isn't used anywhere else.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by:  Stephen D. Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>