librbd: clarify license header

These were meant to say LGPL, but a typo was propagated and it
referred to the non-existent GPL2.1, and also to COPYING which
correctly indicated these files were LGPL2.1.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge pull request #1908 from jdurgin/wip-librbd-license-header

librbd: clarify license header to say LGPL

Everybody who has contributed has already explicitly agreed to relicense RBD to LGPL...except for me, and I am granting it now.

Reviewed-by: Greg Farnum <greg@inktank.com>