netdump: Fix netdumping with INVARIANTS kernels

Correct boneheaded assertion I added in r339501. Mea culpa.

The intent is to notice when an M_WAITOK zone allocation would fail during
netdump, not to prevent all use of mbufs during netdump.

Reviewed by: markj
X-MFC-With: r339501
Differential Revision: https://reviews.freebsd.org/D17957