Please git pull the following tree:
which is based on your for-4.1/drivers branch. Specifically
commit de9ad6d4edb63e0ba5d5aae365fb3565064fc00d "nbd: Return error pointer
This pull has one fix and an cleanup.
Note that David Vrabel in the xen/tip.git tree has other changes for the Xen block
drivers that are related to his grant work - and they do not conflict with this