ext4: Fix sparse warning: Using plain integer as NULL pointer

This patch fixes the warning "Using plain integer as NULL pointer",
generated by sparse, by replacing the offending 0s with NULL.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
index 955cc30..68d92a8 100644
--- a/fs/ext4/page-io.c
+++ b/fs/ext4/page-io.c
@@ -279,9 +279,9 @@
 		BUG_ON(bio_flagged(io->io_bio, BIO_EOPNOTSUPP));
 		bio_put(io->io_bio);
 	}
-	io->io_bio = 0;
+	io->io_bio = NULL;
 	io->io_op = 0;
-	io->io_end = 0;
+	io->io_end = NULL;
 }
 
 static int io_submit_init(struct ext4_io_submit *io,