selinux: fix error codes in cond_read_bool()

The original code always returned -1 (-EPERM) on error.  The new code
returns either -ENOMEM, or -EINVAL or it propagates the error codes from
lower level functions next_entry() or hashtab_insert().

next_entry() returns -EINVAL.
hashtab_insert() returns -EINVAL, -EEXIST, 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>