sys: Mark ACL conversion routines as __result_use_check

Both acl_copy_oldacl_into_acl() and acl_copy_acl_into_oldacl() may fail
in some circumstances (e.g., acl.acl_cnt exceeding the capacity of
OLDACL_MAX_ENTRIES).  This change marks both routines with
__result_use_check, enforcing check for errors by the caller.

Suggested by:	markj
Reviewed by:	markj, emaste
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46254

(cherry picked from commit ef9fc96)
(cherry picked from commit 82fd921)

sys/sys/acl.h

@@ -307,9 +307,9 @@ void			acl_nfs4_compute_inherited_acl(
 			    const struct acl *parent_aclp,
 			    struct acl *child_aclp, mode_t mode,
 			    int file_owner_id, int is_directory);
-int			acl_copy_oldacl_into_acl(const struct oldacl *source,
+int __result_use_check	acl_copy_oldacl_into_acl(const struct oldacl *source,
 			    struct acl *dest);
-int			acl_copy_acl_into_oldacl(const struct acl *source,
+int __result_use_check	acl_copy_acl_into_oldacl(const struct acl *source,
 			    struct oldacl *dest);
 
 /*