MAC: syscalls: mac_label_copyin(): 32-bit compatibility

Needed by the upcoming setcred() system call. More generally, is a step
on the way to support 32-bit compatibility for MAC-related system calls.

Reviewed by: brooks
Approved by: markj (mentor)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47878