qnx4: clean qnx4_fill_super() up

* pass on-disk superblock to qnx4_chkroot() explicitly
* don't leave stale (and unused) pointers in qnx4_super_block
* free stuff in ->kill_sb(); ->put_super() becomes empty and dies
* simplify failure exits

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>