This was a bit silly. We were always creating Ext2FSInode objects when
Ext2FSInode::get_inode() was called. They'd then sit and fatten up the
inode cache forever, despite not representing allocated inodes.
This patch consults the inode bitmap and if get_inode() is called with
an unallocated inode index, we simply cache a nullptr to represent the
fact that this index is unused.
This could be a lot better optimized, it will currently hit the disk for
every new inode index encountered.
8f9542174f