Clean up inode code:

Clean up inode code:

  - Change inode_byte_to_sector() to insist that its argument be a valid
    file offset, and change file_read_at() and file_write_at() to always
    call it with one.

  - Change inode_create() to just create the inode on disk without
    returning a struct inode.  Change return value to bool.  Update
    callers to understand.

  - Integrate alloc_inode() into its one remaining caller.

  - Break inode deallocation into new static function deallocate_inode().

Change bitmap code from using (start...end) to indicate ranges to
using (start, cnt).  Update all callers.  Other minor internal bitmap
cleanups.