Make it more obvious that OVS_KEY_ATTR_MPLS may be an array

Make it more obvious that OVS_KEY_ATTR_MPLS may be an array

Note that OVS_KEY_ATTR_MPLS may be an array of ovs_key_mpls
and that the acceptable length may be restricted by the implementation.

Currently the user-space datapath and proposed kernel datapath
implementation restrict the length to a single element.

Also update the mpls_top_lse name of the element of struct ovs_key_mpls,
as it is an array of LSEs and thus not necessarily just the top LSE.

As requested by Jesse Gross

Cc: Jesse Gross <jesse@nicira.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Jesse Gross <jesse@nicira.com>