ipsec: Drain async ipsec_offload work when destroying a vnet

Re-apply commit e196b12f4d4d. This was reverted by commit 28294dc92476
because it could trigger a deadlock, but the underlying problem there
was fixed in commit f76826b892de.

Reported by: KASAN
Reviewed by: kib
Fixes: ef2a572bf6bd ("ipsec_offload: kernel infrastructure")
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D46483