selinux: fix error codes in symtab_init()

hashtab_create() only returns NULL on allocation failures to -ENOMEM is
appropriate here.

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