libnvdimm/namespace: Fix a potential NULL pointer dereference

[ Upstream commit 55c1fc0af29a6c1b92f217b7eb7581a882e0c07c ]

In case kmemdup fails, the fix goes to blk_err to avoid NULL
pointer dereference.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>