Revert "Illumos 3749 - zfs event processing should work on R/O root filesystems"

This reverts commit b47637ecdc7b647ec5bd9dfca888179eecfaa72d which
introduced a regression in ztest.

$ ./cmd/ztest/ztest -V
5 vdevs, 7 datasets, 23 threads, 300 seconds...

• Error in `/rpool/home/behlendo/src/git/zfs/cmd/ztest/.libs/lt-ztest':

double free or corruption (fasttop): 0x0000000000d339f0 ***