CVE-2024-42314 btrfs: fix extent map use-after-free when adding pages to compressed bio

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix extent map use-after-free when adding pages to compressed bio

At add_ra_bio_pages() we are accessing the extent map to calculate
‘add_size’ after we dropped our reference on the extent map, resulting
in a use-after-free. Fix this by computing ‘add_size’ before dropping our
extent map reference.