net: Fix handling of unmapped user pages in if_getgroup()

We cannot call copyout() while in a net epoch section, unless the user
memory is wired. Use the global ifnet lock to synchronize the accesses
instead.

Reported by: emaste
Reviewed by: zlei
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D57154

(cherry picked from commit 68004e56fdc22c11b4ec680e83309b4ea2bfe13a)