New mount infrastructure for snapshot.

New mount infrastructure for snapshot.

Using the new s_sb_info structure, we add a snapshot number
so we can uniquely identify a snapshot from the superblock and
the 'sget' can be used to find an existing or new superblock.

If it is new, set it up properly as before.

No need to fiddle with 'primary_sb' - we have a ref into it from the
path lookup so it cannot go away, and it shouldn't really matter if it
does.

Signed-off-by: NeilBrown <neilb@suse.de>