selftests/net: Refactor xfrm_fill_key() to use array of structs

A TODO in net/ipsec.c asks to refactor the code in xfrm_fill_key() to
use set/map to avoid manually comparing each algorithm with the "name"
parameter passed to the function as an argument. This patch refactors
the code to create an array of structs where each struct contains the
algorithm name and its corresponding key length.

Signed-off-by: Gautam Menghani <gautammenghani201@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>