fix setting net.isr.dispatch during early boot

[kaktus]

Summary

Fix another collateral damage of rS357614: netisr is initialised way before malloc is available hence it can't use sysctl_handle_string that allocates temporary buffer. Handle that internally in sysctl_netisr_dispatch_policy().

Reported by: delphij
PR: 246114

Test Plan

echo 'net.isr.dispatch="hybrid"' >> /boot/loader.conf && reboot

[kib]

sys/net/netisr.c

361

Why >= ? Should it be > (althought it probably does not matter practically) ?

363

No need for {} for single-statement then block.

[kib]

sys/net/netisr.c

361

I see, you do tmp[len] = 0, which breaks for len == MAXSTR, ignore this question.