nicira-ext: Fix incorrect description of NXM_NX_IPV6_LABEL as non-maskable.

This field has always been fully maskable, but the comment here has always
said that it is non-maskable.

EXT-101.
CC: Jean Tourrilhes <jt@hpl.hp.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>