Accelerated SHA-256 modules no longer need to be manually loaded

[ebiggers]

$ mkfs.btrfs --csum sha256 btrfs.img
[...]
NOTE: you may need to manually load kernel module implementing accelerated SHA256 in case
      the generic implementation is built-in, before mount. Check lsmod or /proc/crypto

This is obsolete in Linux kernel v6.16 and later, since the accelerated SHA-256 code is now always enabled when SHA-256 support is enabled at all. The above warning can be removed when mkfs.btrfs is running on Linux v6.16 and later.

Similar advice also appears in man 5 btrfs.

[kdave]

Thanks for the notice, I'll update the docs and mkfs.