sysv: do not mark superblock dirty on mount

I did not find any docs about this file-system, and I have no possibility
to test my changes. Thus, this is untested.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>