mlx4en(4): Fix wrong mbuf cluster size in mlx4_en_debugnet_init()

This fixes an "invalid cluster size" panic when debugnet is activated.

panic()
m_getzone()
debugnet_mbuf_reinit()
debugnet_any_ifnet_update()
ifhwioctl()
ifioctl()

Submitted by: ae@
PR: 258923
MFC after: 1 week
Sponsored by: NVIDIA Networking