xfrm: Fix memory leak of aead algorithm name

commit 1a6509d9 ("[IPSEC]: Add support for combined mode algorithms")
introduced aead. The function attach_aead kmemdup()s the algorithm
name during xfrm_state_construct().
However this memory is never freed.
Implementation has since been slightly modified in
commit ee5c2317 ("xfrm: Clone states properly on migration")
without resolving this leak.
This patch adds a kfree() call for the aead algorithm name.

Fixes: 1a6509d9 ("[IPSEC]: Add support for combined mode algorithms")
Signed-off-by: Ilan Tayari <ilant@mellanox.com>
Acked-by: Rami Rosen <roszenrami@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>