mnt: Refactor fs_fully_visible into mount_too_revealing

Replace the call of fs_fully_visible in do_new_mount from before the
new superblock is allocated with a call of mount_too_revealing after
the superblock is allocated.   This winds up being a much better location
for maintainability of the code.

The first change this enables is the replacement of FS_USERNS_VISIBLE
with SB_I_USERNS_VISIBLE.  Moving the flag from struct filesystem_type
to sb_iflags on the superblock.

Unfortunately mount_too_revealing fundamentally needs to touch
mnt_flags adding several MNT_LOCKED_XXX flags at the appropriate
times.  If the mnt_flags did not need to be touched the code
could be easily moved into the filesystem specific mount code.

Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

fs/sysfs/mount.c

@@ -42,7 +42,7 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
 		kobj_ns_drop(KOBJ_NS_TYPE_NET, ns);
 	else if (new_sb)
 		/* Userspace would break if executables appear on sysfs */
-		root->d_sb->s_iflags |= SB_I_NOEXEC;
+		root->d_sb->s_iflags |= SB_I_USERNS_VISIBLE | SB_I_NOEXEC;
 
 	return root;
 }
@@ -59,7 +59,7 @@ static struct file_system_type sysfs_fs_type = {
 	.name		= "sysfs",
 	.mount		= sysfs_mount,
 	.kill_sb	= sysfs_kill_sb,
-	.fs_flags	= FS_USERNS_VISIBLE | FS_USERNS_MOUNT,
+	.fs_flags	= FS_USERNS_MOUNT,
 };
 
 int __init sysfs_init(void)