drbd: add missing drbd command names to avoid <NULL> in error messages

cmdname() should map command number to its human readable
representation. The string table was incomplete, though.

Maybe rather do a switch() block, and let the compiler help us
to keep it complete?

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>