Btrfs: cleanup pages properly when ENOMEM in compression

We were freeing non-existent pages which was causing a panic for a user who
was suffering from ENOMEM.  This patch fixes the problem.  Thanks,

Reported-by: Jérôme Poulin <jeromepoulin@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/compression.c

diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index 43d1c5a..c6467aa 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -577,6 +577,7 @@
 	u64 em_start;
 	struct extent_map *em;
 	int ret = -ENOMEM;
+	int faili = 0;
 	u32 *sums;
 
 	tree = &BTRFS_I(inode)->io_tree;
@@ -626,9 +627,13 @@
 	for (pg_index = 0; pg_index < nr_pages; pg_index++) {
 		cb->compressed_pages[pg_index] = alloc_page(GFP_NOFS |
 							      __GFP_HIGHMEM);
-		if (!cb->compressed_pages[pg_index])
+		if (!cb->compressed_pages[pg_index]) {
+			faili = pg_index - 1;
+			ret = -ENOMEM;
 			goto fail2;
+		}
 	}
+	faili = nr_pages - 1;
 	cb->nr_pages = nr_pages;
 
 	add_ra_bio_pages(inode, em_start + em_len, cb);
@@ -713,8 +718,10 @@
 	return 0;
 
 fail2:
-	for (pg_index = 0; pg_index < nr_pages; pg_index++)
-		free_page((unsigned long)cb->compressed_pages[pg_index]);
+	while (faili >= 0) {
+		__free_page(cb->compressed_pages[faili]);
+		faili--;
+	}
 
 	kfree(cb->compressed_pages);
 fail1: