Properly set VNET when nuking recvif from fragment queues.
In theory the eventhandler invoke should be in the same VNET as
the the current interface. We however cannot guarantee that for
all cases in the future.
So before checking if the fragmentation handling for this VNET
is active, switch the VNET to the VNET of the interface to always
get the one we want.
Reviewed by: hselasky
MFC after: 3 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D22153