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>

fs/qnx4/qnx4.h

@@ -10,8 +10,6 @@
 #endif
 
 struct qnx4_sb_info {
-	struct buffer_head	*sb_buf;	/* superblock buffer */
-	struct qnx4_super_block	*sb;		/* our superblock */
 	unsigned int		Version;	/* may be useful */
 	struct qnx4_inode_entry	*BitMap;	/* useful */
 };