btrfs: fix bytes_may_use underflow in prealloc error condtition

I hit the following warning while running my error injection stress
testing:

  WARNING: CPU: 3 PID: 1453 at fs/btrfs/space-info.h:108 btrfs_free_reserved_data_space_noquota+0xfd/0x160 [btrfs]
  RIP: 0010:btrfs_free_reserved_data_space_noquota+0xfd/0x160 [btrfs]
  Call Trace:
  btrfs_free_reserved_data_space+0x4f/0x70 [btrfs]
  __btrfs_prealloc_file_range+0x378/0x470 [btrfs]
  elfcorehdr_read+0x40/0x40
  ? elfcorehdr_read+0x40/0x40
  ? btrfs_commit_transaction+0xca/0xa50 [btrfs]
  ? dput+0xb4/0x2a0
  ? btrfs_log_dentry_safe+0x55/0x70 [btrfs]
  ? btrfs_sync_file+0x30e/0x420 [btrfs]
  ? do_fsync+0x38/0x70
  ? __x64_sys_fdatasync+0x13/0x20
  ? do_syscall_64+0x5b/0x1b0
  ? entry_SYSCALL_64_after_hwframe+0x44/0xa9

This happens if we fail to insert our reserved file extent.  At this
point we've already converted our reservation from ->bytes_may_use to
->bytes_reserved.  However once we break we will attempt to free
everything from [cur_offset, end] from ->bytes_may_use, but our extent
reservation will overlap part of this.

Fix this problem by adding ins.offset (our extent allocation size) to
cur_offset so we remove the actual remaining part from ->bytes_may_use.

I validated this fix using my inject-error.py script

python inject-error.py -o should_fail_bio -t cache_save_setup -t \
	__btrfs_prealloc_file_range \
	-t insert_reserved_file_extent.constprop.0 \
	-r "-5" ./run-fsstress.sh

where run-fsstress.sh simply mounts and runs fsstress on a disk.

CC: stable@vger.kernel.org # 4.4+
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -9824,6 +9824,7 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
 	struct btrfs_root *root = BTRFS_I(inode)->root;
 	struct btrfs_key ins;
 	u64 cur_offset = start;
+	u64 clear_offset = start;
 	u64 i_size;
 	u64 cur_bytes;
 	u64 last_alloc = (u64)-1;
@@ -9858,6 +9859,15 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
 				btrfs_end_transaction(trans);
 			break;
 		}
+
+		/*
+		 * We've reserved this space, and thus converted it from
+		 * ->bytes_may_use to ->bytes_reserved.  Any error that happens
+		 * from here on out we will only need to clear our reservation
+		 * for the remaining unreserved area, so advance our
+		 * clear_offset by our extent size.
+		 */
+		clear_offset += ins.offset;
 		btrfs_dec_block_group_reservations(fs_info, ins.objectid);
 
 		last_alloc = ins.offset;
@@ -9937,9 +9947,9 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
 		if (own_trans)
 			btrfs_end_transaction(trans);
 	}
-	if (cur_offset < end)
-		btrfs_free_reserved_data_space(inode, NULL, cur_offset,
-			end - cur_offset + 1);
+	if (clear_offset < end)
+		btrfs_free_reserved_data_space(inode, NULL, clear_offset,
+			end - clear_offset + 1);
 	return ret;
 }