if_vlan: Stop checking for failures from malloc(M_WAITOK)

Fixes: b08d611de835 fix vlan locking to permit sx acquisition in ioctl calls
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45852

(cherry picked from commit 07b16b1e2aeab0b30f68a013de31a4c322a61246)
(cherry picked from commit 67971181687f0475dcf72b42d065364efea96251)