bcachefs: twf: convert bch2_stdio_redirect_readline() to darray

bcachefs: twf: convert bch2_stdio_redirect_readline() to darray

We now read the line from the buffer atomically, which means we have to
allow the buffer to grow past STDIO_REDIRECT_BUFSIZE if we're waiting
for a full line - this behaviour is necessary for
stdio_redirect_readline_timeout() in the next patch.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>