Almost all of the metadata goes in under the objectid of a given file or directory. There are
exceptions like file data checksums and the records about which extents are allocated, but they
go into their own dedicated btree.
The end result is that almost all of the 2^64 space can be used for inodes. There are a few
special inodes, like one that is used to prevent orphan inodes after a crash, so we carve out the
last 256 inode numbers for specialized metadata. We also carve out the first 256 inode numbers
for special use by the backref code.