Remove gratuitous copyouts of unchanged struct mac.

The get operations change the data pointed to by the structure, but do
not update the contents of the struct.

Mark the struct mac arguments of mac_[gs]etsockopt_*label() and
mac_check_structmac_consistent() const to prevent this from changing
in the future.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D14488