Add 'filesys' pointer to lafs_inode

Add 'filesys' pointer to lafs_inode

This is the first patch in a series to switch from having one
struct super_block for each subset filesystem, to only having one
for each snapshot.  i.e. one which is writable and some number
which are read-only snapshots.
The struct super_block described the primary filesystem and
all of the subset filesystems.

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