linux: setgroups(): Avoid allocation under the process lock

This was missed in commit 838d9858251e ("Rework the credential code to
support larger values of NGROUPS (...)").

No functional change (intended).

MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52278

(cherry picked from commit b7a87d7c0dd19016e82f5d33f2c5ec90479c31fe)