Fix generation of non-standard "scan" in object directory.

Commit 1421c89 expanded the size of a zbookmark_t from 24 to 25 64-bit
values which similarly expands the size of the "scan" entry in the pool's
object directory and causes the pool to become un-importable by other
OpenZFS implementations.

This commit replaces the zbookmark_t member of dsl_scan_phys_t with 4
new members representing the contents of the zbookmark_t which insulates
the physical layout from changes to that structure.

NOTE: The same could be done with the ddt_bookmark_t in dsl-scan_phys_t.

Add a comptibility shim to dsl_scan_init() to support importing broken
pools that have the larger-than-normal "scan" entry.

Fixes #2094