iflib: Unlock ctx lock around call to ether_ifattach()

Panic occurs during loading driver using kldload. It exists since netlink is
enabled. There is problem with double locking ctx. This fix allows to call
ether_ifattach() without locked ctx.

Signed-off-by: Eric Joyner <erj@FreeBSD.org>

PR: 271768
Reviewed by: erj@, jhb@
MFC after: 1 day
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D40557