pmem: kill __pmem address space

The __pmem address space was meant to annotate codepaths that touch
persistent memory and need to coordinate a call to wmb_pmem().  Now that
wmb_pmem() is gone, there is little need to keep this annotation.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>