bcachefs: logged ops only use inum 0 of logged ops btree

bcachefs: logged ops only use inum 0 of logged ops btree

we wish to use the logged ops btree for other items that aren't strictly
logged ops: cursors for inode allocation

There's no reason to create another cached btree for inode allocator
cursors - so reserve different parts of the keyspace for different
purposes.

Older versions will ignore or delete the cursors.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>