btrfs: update stale comment for nowait direct IO writes

If when doing a direct IO write we need to fallback to buffered IO, we
this comment at btrfs_direct_write() that says we can't directly fallback
to buffered IO if we have a NOWAIT iocb, because we have no support for
NOWAIT buffered writes. That is not true anymore, as support for NOWAIT
buffered writes was added recently in commit 926078b2 ("btrfs: enable
nowait async buffered writes").

However we still can't fallback to a buffered write in case we have a
NOWAIT iocb, because we'll need to flush delalloc and wait for it to
complete after doing the buffered write, and that can block for several
reasons, the main reason being waiting for IO to complete.

So update the comment to mention all that.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/file.c

@@ -1575,8 +1575,8 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
 	/*
 	 * If we are in a NOWAIT context, then return -EAGAIN to signal the caller
 	 * it must retry the operation in a context where blocking is acceptable,
-	 * since we currently don't have NOWAIT semantics support for buffered IO
-	 * and may block there for many reasons (reserving space for example).
+	 * because even if we end up not blocking during the buffered IO attempt
+	 * below, we will block when flushing and waiting for the IO.
 	 */
 	if (iocb->ki_flags & IOCB_NOWAIT) {
 		err = -EAGAIN;