MFV r260155:

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@8b36997aa24d9817807faa4efa301ac9c07a2b78

MFC after: 2 weeks