ipv6: sr: fix memleak in seg6_hmac_init_algo

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.