ipv6: sr: fix memleak in seg6_hmac_init_algo

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.

Fixes: bf355b8d2c30 ("ipv6: sr: add core files for SR HMAC support")
Reported-by: Sabrina Dubroca <sd@queasysnail.net>
Closes: https://lore.kernel.org/netdev/Zj3bh-gE7eT6V6aH@hog/
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://lore.kernel.org/r/20240517005435.2600277-1-liuhangbin@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>