Widen EPOCH(9) usage in netisr.

Software interrupt handlers are allowed to sleep. In swi_net() there
is a read lock behind NETISR_RLOCK() which in turn ends up calling
msleep() which means the whole of swi_net() cannot be protected by an
EPOCH(9) section. By default the NETISR_LOCKING feature is disabled.

This issue was introduced by r357004. This is a preparation step for
replacing the functionality provided by r357004.

Found by: kib@
Sponsored by: Mellanox Technologies