Manage: when re-adding, do check avail size if ->sb cannot be found.

Manage: when re-adding, do check avail size if ->sb cannot be found.

avail_size1 requires ->sb, so we must only call it if ->sb
was loaded.

If ->sb wasn't loaded, then we are only proceding on the basis that
the kernel might be able to work something out - we don't need to
do any tests on size.

Reported-by: Christoffer Hammarström <christoffer.hammarstrom@linuxgods.com>
Signed-off-by: NeilBrown <neilb@suse.de>
URL: https://bugs.debian.org/784874