af_key: make use of BUG_ON macro

fix below warnings reported by coccicheck

net/key/af_key.c:932:2-5: WARNING: Use BUG_ON instead of if condition
followed by BUG.
net/key/af_key.c:948:2-5: WARNING: Use BUG_ON instead of if condition
followed by BUG.

Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>