mlx4, mlx5: Eliminate redundent NULL check for packet filter

mlx4 and mlx5 are Ethernet devices and ether_ifattach() does an
unconditional bpfattach(). From commit 16d878cc99ef [1] and on, we
should not check ifp->if_bpf to tell us whether or not we have any bpf
peers that might be interested in receiving packets. And since commit
2b9600b4497b [2], ifp->if_bpf can not be NULL even after the network
interface has been detached.

No functional change intended.

1. 16d878cc99ef Fix the following bpf(4) race condition which can result in a panic
2. 2b9600b4497b Add dead_bpf_if structure, that should be used as fake bpf_if during ifnet detach

Reviewed by: kp, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45196

(cherry picked from commit 2439ae948352766f6b993c5103a4c516376bec28)
(cherry picked from commit d98ced6588b522e8833888bc99b7b9468312b221)