When we encounter an I/O error on a piece of metadata while deleting
a file system or zvol, we don't update the bptree_entry_phys_t's
bookmark. This would lead to double free of bp's which will lead to
space map corruption.
Instead of tolerating and allowing the corruption, panic immediately.
See Illumos #4390 for more details.
4391 panic system rather than corrupting pool if we hit bug 4390
Illumos/illumos-gate@8b36997aa2
MFC after: 2 weeks
7c88e58f46