linux: setgroups16(): Pre-extend the groups array

For the size we know we will need in the end.

No functional change (intended).

MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52279

(cherry picked from commit b1326edddb302a28fc9418ab60df6210513cc07c)