xfrm: Remove rebundant address family checking

present_and_same_family has checked addresses family validness for both
SADB_EXT_ADDRESS_SRC and SADB_EXT_ADDRESS_DST in the beginning.
Thereafter pfkey_sadb_addr2xfrm_addr doesn't need to do the checking again.

Signed-off-by: Fan Du <fan.du@windriver.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>