checkpin: don't hold references on primary superblock.

checkpin: don't hold references on primary superblock.

This isn't really a need to hold a reference on the primary
superblock as everything else does and when the last reference goes
it will stop the cleaner so these references won't be needed any.

This is only an interim solution - we will be removing the
multiple superblocks soon and all this will go away.

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