mount: handle mnt_flags and sb_flags separatly (v4)

They both can container the MS_READONLY flag. And in one case it will be
read-only bind-mount and in another case it will be read-only
super-block.

v2: set mnt and sb for one call of mount() when it's posiable
v3: return a comment which was deleted by mistake
v4: Fix the sentense about restoring mnt flags
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>