batman-adv: properly check pskb_may_pull return value

batman-adv: properly check pskb_may_pull return value

[ Upstream commit f1791425cf0bcda43ab9a9a37df1ad3ccb1f6654 ]

pskb_may_pull() returns 1 on success and 0 in case of failure,
therefore checking for the return value being negative does
not make sense at all.

This way if the function fails we will probably read beyond the current
skb data buffer. Fix this by doing the proper check.

Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>