mount: check for unsuccessful read-only bind mounts

mount: check for unsuccessful read-only bind mounts

Linux kernel allows to use MS_RDONLY together with MS_BIND,
unfortunately the MS_RDONLY is silently ignored and the target
mountpoint is still read-write. Then we have 'ro' in mtab and 'rw' in
/proc/mounts.

This patch checks for this situation by access(2) or futimens(2)
(change atime) and mtab is properly updated and user informed.

Reported-by: Terry Burton <tez@terryburton.co.uk>
Signed-off-by: Karel Zak <kzak@redhat.com>