CVE-2024-39489

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

ipv6: sr: fix memleak in seg6_hmac_init_algo

seg6_hmac_init_algo returns without cleaning up the previous allocations
if one fails, so it's going to leak all that memory and the crypto tfms.

Update seg6_hmac_exit to only free the memory when allocated, so we can
reuse the code directly.