ocfs2: Fix invalid assertion during write on 64k pages

The write path code intends to bug if a math error (or unhandled case)
results in a write outside of the current cluster boundaries. The actual
BUG_ON() statements however are incorrect, leading to a crash on kernels
with 64k page size. Fix those by checking against the right variables.

Also, move the assertions higher up within the functions so that they trip
*before* the code starts to mark buffers.
Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>

fs/ocfs2/aops.c

@@ -798,6 +798,11 @@ int ocfs2_map_and_write_splice_data(struct inode *inode,
 	}
 	to = from + bytes;
 
+	BUG_ON(from > PAGE_CACHE_SIZE);
+	BUG_ON(to > PAGE_CACHE_SIZE);
+	BUG_ON(from < cluster_start);
+	BUG_ON(to > cluster_end);
+
 	if (wc->w_this_page_new)
 		ret = ocfs2_map_page_blocks(wc->w_this_page, p_blkno, inode,
 					    cluster_start, cluster_end, 1);
@@ -809,11 +814,6 @@ int ocfs2_map_and_write_splice_data(struct inode *inode,
 		goto out;
 	}
 
-	BUG_ON(from > PAGE_CACHE_SIZE);
-	BUG_ON(to > PAGE_CACHE_SIZE);
-	BUG_ON(from > osb->s_clustersize);
-	BUG_ON(to > osb->s_clustersize);
-
 	src = buf->ops->map(sp->s_pipe, buf, 1);
 	dst = kmap_atomic(wc->w_this_page, KM_USER1);
 	memcpy(dst + from, src + src_from, bytes);
@@ -890,6 +890,11 @@ int ocfs2_map_and_write_user_data(struct inode *inode,
 
 	to = from + bytes;
 
+	BUG_ON(from > PAGE_CACHE_SIZE);
+	BUG_ON(to > PAGE_CACHE_SIZE);
+	BUG_ON(from < cluster_start);
+	BUG_ON(to > cluster_end);
+
 	if (wc->w_this_page_new)
 		ret = ocfs2_map_page_blocks(wc->w_this_page, p_blkno, inode,
 					    cluster_start, cluster_end, 1);
@@ -901,11 +906,6 @@ int ocfs2_map_and_write_user_data(struct inode *inode,
 		goto out;
 	}
 
-	BUG_ON(from > PAGE_CACHE_SIZE);
-	BUG_ON(to > PAGE_CACHE_SIZE);
-	BUG_ON(from > osb->s_clustersize);
-	BUG_ON(to > osb->s_clustersize);
-
 	dst = kmap(wc->w_this_page);
 	memcpy(dst + from, bp->b_src_buf + src_from, bytes);
 	kunmap(wc->w_this_page);