if_stf: enable use in vnet jails

The cloner must be per-vnet so that cloned interfaces get destroyed when
the vnet goes away. Otherwise we fail assertions in vnet_if_uninit():

panic: vnet_if_uninit:475 tailq &V_ifnet=0xfffffe01665fe070 not empty
cpuid = 19
time = 1636107064
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe015d0cac60
vpanic() at vpanic+0x187/frame 0xfffffe015d0cacc0
panic() at panic+0x43/frame 0xfffffe015d0cad20
vnet_if_uninit() at vnet_if_uninit+0x7b/frame 0xfffffe015d0cad30
vnet_destroy() at vnet_destroy+0x170/frame 0xfffffe015d0cad60
prison_deref() at prison_deref+0x9b0/frame 0xfffffe015d0cadd0
sys_jail_remove() at sys_jail_remove+0x119/frame 0xfffffe015d0cae00
amd64_syscall() at amd64_syscall+0x12e/frame 0xfffffe015d0caf30
fast_syscall_common() at fast_syscall_common+0xf8/frame 0xfffffe015d0caf30

• syscall (508, FreeBSD ELF64, sys_jail_remove), rip = 0x8011e920a, rsp = 0x7fffffffe788, rbp = 0x7fffffffe810 ---

KDB: enter: panic

MFC after: 3 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D32849

(cherry picked from commit 8e45fed3aefd00ff544efce8fedeba5e099d3d01)