wext: optimise, comment and fix event sending

The current function for sending events first allocates the
event stream buffer, and then an skb to copy the event stream
into. This can be done in one go. Also, the current function
leaks kernel data to userspace in a 4 uninitialised bytes,
initialise those explicitly. Finally also add a few useful
comments, as opposed to the current comments.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>