nbd: Bump nbd to version 2.9.15 and remove klcc issue.

This patch bumps the version of nbd to 2.9.15.
The new version avoids problems with klcc installed on the host system
and also includes two former buildroot patches:
- nbd-fix-build-with-disable-lfs.patch
- nbd-server-fix-read-eagain.patch

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>