Wrong calcultation in nla_reserve

There seams to be an error in the calculation of needed space for the message in nla_reserve. The current size of the message is counted twice: Once in NLMSG_ALIGN, once in the condition below.
This causes nla_put_* calls to be rejected if the allocation size of the message has been strictly calculated by the caller.

Signed-off-by: Thomas Graf <tgraf@suug.ch>