vfs: fix mounting a filesystem with i_version

The mount i_version flag is not enabled in the new sb_flags.  This patch
adds the missing SB_I_VERSION flag.

Fixes: e462ec50 "VFS: Differentiate mount flags (MS_*) from internal
       superblock flags"
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>