fscrypt: use HMAC-SHA512 library for HKDF

For the HKDF-SHA512 key derivation needed by fscrypt, just use the
HMAC-SHA512 library functions directly.  These functions were introduced
in v6.17, and they provide simple and efficient direct support for
HMAC-SHA512.  This ends up being quite a bit simpler and more efficient
than using crypto/hkdf.c, as it avoids the generic crypto layer:

- The HMAC library can't fail, so callers don't need to handle errors
- No inefficient indirect calls
- No inefficient and error-prone dynamic allocations
- No inefficient and error-prone loading of algorithm by name
- Less stack usage

Benchmarks on x86_64 show that deriving a per-file key gets about 30%
faster, and FS_IOC_ADD_ENCRYPTION_KEY gets nearly twice as fast.

The only small downside is the HKDF-Expand logic gets duplicated again.
Then again, even considering that, the new fscrypt_hkdf_expand() is only
7 lines longer than the version that called hkdf_expand().  Later we
could add HKDF support to lib/crypto/, but for now let's just do this.

Link: https://lore.kernel.org/r/20250906035913.1141532-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

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