ext4: fix ext4_discard_allocated_blocks() if we can't allocate the pa struct

If there is a failure while allocating the preallocation structure, a
number of blocks can end up getting marked in the in-memory buddy
bitmap, and then not getting released.  This can result in the
following corruption getting reported by the kernel:

EXT4-fs error (device sda3): ext4_mb_generate_buddy:758: group 1126,
12793 clusters in bitmap, 12729 in gd

In that case, we need to release the blocks using mb_free_blocks().

Tested: fs smoke test; also demonstrated that with injected errors,
	the file system is no longer getting corrupted

Google-Bug-Id: 16657874

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org

fs/ext4/mballoc.c

@@ -3217,8 +3217,27 @@ static void ext4_mb_collect_stats(struct ext4_allocation_context *ac)
 static void ext4_discard_allocated_blocks(struct ext4_allocation_context *ac)
 {
 	struct ext4_prealloc_space *pa = ac->ac_pa;
+	struct ext4_buddy e4b;
+	int err;
 
-	if (pa && pa->pa_type == MB_INODE_PA)
+	if (pa == NULL) {
+		err = ext4_mb_load_buddy(ac->ac_sb, ac->ac_f_ex.fe_group, &e4b);
+		if (err) {
+			/*
+			 * This should never happen since we pin the
+			 * pages in the ext4_allocation_context so
+			 * ext4_mb_load_buddy() should never fail.
+			 */
+			WARN(1, "mb_load_buddy failed (%d)", err);
+			return;
+		}
+		ext4_lock_group(ac->ac_sb, ac->ac_f_ex.fe_group);
+		mb_free_blocks(ac->ac_inode, &e4b, ac->ac_f_ex.fe_start,
+			       ac->ac_f_ex.fe_len);
+		ext4_unlock_group(ac->ac_sb, ac->ac_f_ex.fe_group);
+		return;
+	}
+	if (pa->pa_type == MB_INODE_PA)
 		pa->pa_free += ac->ac_b_ex.fe_len;
 }