FS-Cache: Increase reference of parent after registering, netfs success

FS-Cache: Increase reference of parent after registering, netfs success

If netfs exist, fscache should not increase the reference of parent's
usage and n_children, otherwise, never be decreased.

v2: thanks David's suggest,
 move increasing reference of parent if success
 use kmem_cache_free() freeing primary_index directly

v3: don't move "netfs->primary_index->parent = &fscache_fsdef_index;"

Cc: [email protected] # v2.6.30+
Signed-off-by: Kinglong Mee <[email protected]>
Signed-off-by: David Howells <[email protected]>
Signed-off-by: Al Viro <[email protected]>