sysctl: fix setting net.isr.dispatch during early boot

Fix another collateral damage of r357614: 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().

PR: 246114
Reported by: delphij
Reviewed by: kib
Approved by: kib (mentor)
Differential Revision: https://reviews.freebsd.org/D24858