btrfs: fix state->private cast on 32 bit machines

Suppress the following warning displayed on building 32bit (i686) kernel.

===============================================================================
...
   CC [M]  fs/btrfs/extent_io.o
fs/btrfs/extent_io.c: In function ‘btrfs_free_io_failure_record’:
fs/btrfs/extent_io.c:2193:13: warning: cast to pointer from integer of
different size [-Wint-to-pointer-cast]
    failrec = (struct io_failure_record *)state->private;
...
===============================================================================
Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Reported-by: Chris Murphy <chris@colorremedies.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/extent_io.c

@@ -2190,7 +2190,7 @@ void btrfs_free_io_failure_record(struct inode *inode, u64 start, u64 end)
 
 		next = next_state(state);
 
-		failrec = (struct io_failure_record *)state->private;
+		failrec = (struct io_failure_record *)(unsigned long)state->private;
 		free_extent_state(state);
 		kfree(failrec);