Read: There was a missing refcount on the closure.
Write: Unlike in the Linux kernel, we need to manually update the size
and timestamps, otherwise the operation will appear complete when viewed
via the cached inode, but after unmounting and mounting again the write
will be undone.
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.
