net: xfrm_user: use BUG_ON instead of if condition followed by BUG

Use BUG_ON instead of if condition followed by BUG.

This issue was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>