udf: Don't modify filesystem for read-only mounts

When read-write mount of a filesystem is requested but we find out we
can mount the filesystem only in read-only mode, we still modify
LVID in udf_close_lvid(). That is both unnecessary and contrary to
expectation that when we fall back to read-only mount we don't modify
the filesystem.

Make sure we call udf_close_lvid() only if we called udf_open_lvid() so
that filesystem gets modified only if we verified we are allowed to
write to it.

Reported-by: Karel Zak <kzak@redhat.com>
Signed-off-by: Jan Kara <jack@suse.com>