selinux: fix error codes in cond_read_node()

Originally cond_read_node() returned -1 (-EPERM) on errors which was
incorrect.  Now it either propagates the error codes from lower level
functions next_entry() or cond_read_av_list() or it returns -ENOMEM or
-EINVAL.

next_entry() returns -EINVAL.
cond_read_av_list() returns -EINVAL or -ENOMEM.

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>