ipv6: sr: Use HMAC-SHA1 and HMAC-SHA256 library functions

ipv6: sr: Use HMAC-SHA1 and HMAC-SHA256 library functions

Use the HMAC-SHA1 and HMAC-SHA256 library functions instead of
crypto_shash.  This is simpler and faster.  Pre-allocating per-CPU hash
transformation objects and descriptors is no longer needed, and a
microbenchmark on x86_64 shows seg6_hmac_compute() (with HMAC-SHA256)
dropping from ~2494 cycles to ~1978 cycles, a 20% improvement.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Link: https://patch.msgid.link/20250824013644.71928-2-ebiggers@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>