bcachefs: bcachefs_metadata_version_persistent_inode_cursors

bcachefs: bcachefs_metadata_version_persistent_inode_cursors

Persistent cursors for inode allocation.

A free inodes btree would add substantial overhead to inode allocation
and freeing - a "next num to allocate" cursor is always going to be
faster.

We just need it to be persistent, to avoid scanning the inodes btree
from the start on startup.

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