jfs: Fix a typo in function jfs_umount

When closing the block allocation map, an incorrect pointer
was NULL'ed. This commit fixes that.

Signed-off-by: Oleg Kanatov <[e-mail]>
Signed-off-by: Dave Kleikamp <[e-mail]>

fs/jfs/jfs_umount.c

···
 	dbUnmount(ipbmap, 0);
 
 	diFreeSpecial(ipbmap);
-	sbi->ipimap = NULL;
+	sbi->ipbmap = NULL;
 
 	/*
 	 * Make sure all metadata makes it to disk before we mark