net/sched: act_mpls: ensure LSE is pullable before reading it

when 'act_mpls' is used to mangle the LSE, the current value is read from
the packet dereferencing 4 bytes at mpls_hdr(): ensure that the label is
contained in the skb "linear" area.

Found by code inspection.

v2:
 - use MPLS_HLEN instead of sizeof(new_lse), thanks to Jakub Kicinski

Fixes: 2a2ea508 ("net: sched: add mpls manipulation actions to TC")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Guillaume Nault <gnault@redhat.com>
Link: https://lore.kernel.org/r/3243506cba43d14858f3bd21ee0994160e44d64a.1606987058.git.dcaratti@redhat.com


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