FS-Cache: Don't override netfs's primary_index if registering failed

commit b130ed59 upstream.

Only override netfs->primary_index when registering success.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
[bwh: Backported to 3.2: no n_active or flags fields in fscache_cookie]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>