sparse: fix misplaced __pmem definition

sparse: fix misplaced __pmem definition

Orabug: 22913653

Move the definition of __pmem outside of CONFIG_SPARSE_RCU_POINTER to fix:

drivers/nvdimm/pmem.c:198:17: sparse: too many arguments for function __builtin_expect
drivers/nvdimm/pmem.c:36:33: sparse: expected ; at end of declaration
drivers/nvdimm/pmem.c:48:21: sparse: void declaration

...due to __pmem failing to be defined in some configurations when
CONFIG_SPARSE_RCU_POINTER=y.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
(cherry picked from commit 31f02455455d405320e2f749696bef4e02903b35)
Signed-off-by: Dan Duval <dan.duval@oracle.com>