ext4: quiet sparse noise about plain integer as NULL pointer

The third parameter to ext4_free_blocks is a struct buffer_head *.  This
parameter should be NULL not 0.

This quiets the sparse noise:

warning: Using plain integer as NULL pointer
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/extents.c

@@ -2291,7 +2291,7 @@ static int ext4_remove_blocks(handle_t *handle, struct inode *inode,
 		start = ext4_ext_pblock(ex);
 
 		ext_debug("free first %u blocks starting %llu\n", num, start);
-		ext4_free_blocks(handle, inode, 0, start, num, flags);
+		ext4_free_blocks(handle, inode, NULL, start, num, flags);
 
 	} else {
 		printk(KERN_INFO "strange request: removal(2) "