net: skbuff: ensure LSE is pullable before decrementing the MPLS ttl

skb_mpls_dec_ttl() reads the LSE without ensuring that it is contained in
the skb "linear" area. Fix this calling pskb_may_pull() before reading the
current ttl.

Found by code inspection.

Fixes: 2a2ea508 ("net: sched: add mpls manipulation actions to TC")
Reported-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Link: https://lore.kernel.org/r/53659f28be8bc336c113b5254dc637cc76bbae91.1606987074.git.dcaratti@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>