pmem: reduce kmap_atomic sections to the memcpys only

pmem_do_bvec used to kmap_atomic at the begin, and only unmap at the
end. Things like nvdimm_clear_poison may want to do nvdimm subsystem
bookkeeping operations that may involve taking locks or doing memory
allocations, and we can't do that from the atomic context. Reduce the
atomic context to just what needs it - the memcpy to/from pmem.

Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>