Also adds partial . and .. support, and implements ENOTDIR error.
The source of this problem was due to the off-by-one interface expected
by readdir. Each directory entry contains a pointer to next, so it
cannot be emitted to the user until the next entry is actually read.
Returning the current position + 1 is insufficient, because the user
will just retry cur + 1 as the start position to see if they've reached
the end of the directory.
Since directory entries are keyed by a 64-bit hash in bcachefs, the
result was that the user would call readdir over and over with what it
believed the next pointer to be until "cur + 1" reached some enormous
64-bit value related to the hash for the "lost+found" entry.
