net-next: skbuff: refactor pskb_pull

pskb_may_pull already contains all of the checks performed by
pskb_pull.
Use pskb_may_pull for validation in pskb_pull, eliminating the
duplication and making __pskb_pull obsolete.
Replace __pskb_pull with pskb_pull where applicable.

Signed-off-by: Richard Gobert <richardbgobert@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>