autofs: clear O_NONBLOCK on the pipe

autofs does not expect the pipe it is given to have O_NONBLOCK set -
specifically if __kernel_write() in autofs_write() returns -EAGAIN, this
is treated as a fatal error and the pipe is closed.

For safety autofs should, therefore, clear the O_NONBLOCK flag.

Releases of systemd prior to 8th February 2019 used
  pipe2(p, O_NONBLOCK|O_CLOEXEC)
and thus (inadvertently) set this flag.

Link: http://lkml.kernel.org/r/154993550902.3321.1183632970046073478.stgit@pluto-themaw-netSigned-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/autofs/autofs_i.h

@@ -216,6 +216,8 @@ static inline int autofs_prepare_pipe(struct file *pipe)
 		return -EINVAL;
 	/* We want a packet pipe */
 	pipe->f_flags |= O_DIRECT;
+	/* We don't expect -EAGAIN */
+	pipe->f_flags &= ~O_NONBLOCK;
 	return 0;
 }