nixos/btrfs: use sha256 instead of sha256_generic

Upstream dropped `sha256_generic` in https://github.com/torvalds/linux/commit/e96cb9507f2d8ba150d417dcd283204564945831

Quoting from upstream:

> sha256_blocks_generic() is moved from lib/crypto/sha256-generic.c into
lib/crypto/sha256.c. It's now a static function marked with
__maybe_unused, so the compiler automatically eliminates it in any
cases where it's not used.

Co-authored-by: dramforever <[e-mail]>

nixos/modules/tasks/filesystems/btrfs.nix

···
         # Needed for mounting filesystems with new checksums
         "xxhash_generic"
         "blake2b_generic"
-        "sha256_generic" # Should be baked into our kernel, just to be sure
+
+        # `sha256` is always available, whereas `sha256_generic` is not available from 6.17 onwards
+        "sha256" # Should be baked into our kernel, just to be sure
       ];
 
       boot.initrd.extraUtilsCommands = mkIf (!config.boot.initrd.systemd.enable) ''