btrfs: fix extent map use-after-free when handling missing device in read_one_chunk

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

btrfs: fix extent map use-after-free when handling missing device in read_one_chunk

Store the error code before freeing the extent_map. Though it's reference counted structure, in that function it's the first and last allocation so this would lead to a potential use-after-free.

The error can happen eg. when chunk is stored on a missing device and the degraded mount option is missing.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=216721