usb-mtp: use O_NOFOLLOW and O_CLOEXEC.

usb-mtp: use O_NOFOLLOW and O_CLOEXEC.

Open files and directories with O_NOFOLLOW to avoid symlinks attacks.
While being at it also add O_CLOEXEC.

usb-mtp only handles regular files and directories and ignores
everything else, so users should not see a difference.

Because qemu ignores symlinks, carrying out a successful symlink attack
requires swapping an existing file or directory below rootdir for a
symlink and winning the race against the inotify notification to qemu.

Fixes: CVE-2018-16872
Cc: Prasad J Pandit <ppandit@redhat.com>
Cc: Bandan Das <bsd@redhat.com>
Reported-by: Michael Hanselmann <public@hansmi.ch>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Michael Hanselmann <public@hansmi.ch>
Message-id: 20181213122511.13853-1-kraxel@redhat.com