curve: curve25519_tfm_alloc/curve25519_tfm_free helpers

Facilitate allocation and destruction of crypto objects through
common helper functions.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
curve: free buffers in reverse order to allocation

First destroy spinlocks, then free the encryption and decryption
buffers.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>