This changes storage.mount() to require that a mount point exist
on the parent file system.
A bug in background tasks is also fixed where the function
parameter is cleared on pending callbacks during "reset".
Disk usage is shown on the directory listing and changes based on
the mounted file system. Writable is also loaded per-directory.
Fixes#8108. Fixes#8690. Fixes#8107.
This commit adds helper functions to call readblocks/writeblocks with a
fourth argument, the byte offset within a block.
Although the mp_vfs_blockdev_t struct has grown here by 2 machine words, in
all current uses of this struct within this repository it still fits within
the same number of GC blocks.
