libnvdimm/btt: Fix a kmemdup failure check

[ Upstream commit 486fa92df4707b5df58d6508728bdb9321a59766 ]

In case kmemdup fails, the fix releases resources and returns to
avoid the NULL pointer dereference.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>