fs/reiserfs/inode.c: replace 0 by NULL for pointers

Fix sparse warning:

  fs/reiserfs/inode.c:2769:19: warning: Using plain integer as NULL pointer
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/reiserfs/inode.c

@@ -2766,7 +2766,7 @@ static int reiserfs_write_begin(struct file *file,
 	int old_ref = 0;
 
  	inode = mapping->host;
-	*fsdata = 0;
+	*fsdata = NULL;
  	if (flags & AOP_FLAG_CONT_EXPAND &&
  	    (pos & (inode->i_sb->s_blocksize - 1)) == 0) {
  		pos ++;