selinux: Fix a problem in security_netlbl_sid_to_secattr()

Currently when SELinux fails to allocate memory in
security_netlbl_sid_to_secattr() the NetLabel LSM domain field is set to
NULL which triggers the default NetLabel LSM domain mapping which may not
always be the desired mapping.  This patch fixes this by returning an error
when the kernel is unable to allocate memory.  This could result in more
failures on a system with heavy memory pressure but it is the "correct"
thing to do.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Acked-by: James Morris <jmorris@namei.org>