Fix generation of non-standard "scan" and "bptree_obj" 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 renames "struct zbookmark" to "struct zbookmark_phys" since
it is related to an on-disk format and adds a new "struct zbookmark" that
contains the former as its first member.  The effect is that the "struct
zbookmark" items written to the object directory in both the "scan" and
"bptree_obj" entries contain only the correct subset of the bookmark.

Fixes #2094