netinet: enter epoch in garp_rexmit()

garp_rexmit() is a callback, so is not in net_epoch, which
arprequest_internal() expects.
Enter and exit the net_epoch.

PR: 284073
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 38fdcca05d09b4d5426a253d3c484f9481a73ac2)