MAC: syscalls: Factor out common label copy-in code

Besides simplifying existing code, this will later enable the new
setcred() system call to copy MAC labels.

MFC after: 2 weeks
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46904

(cherry picked from commit 2e593dd3b5e1c515d57b3d3f929e544a6622b04a)