libnvdimm, pfn: move 'memory mode' indication to sysfs

'Memory mode' is defined as the capability of a DAX mapping to be the
source/target of DMA and other "direct I/O" scenarios.  While it
currently requires allocating 'struct page' for each page frame of
persistent memory in the namespace it will not always be the case.  Work
continues on reducing the kernel's dependency on 'struct page'.

Let's not maintain a suffix that is expected to lose meaning over time.
In other words a future 'raw mode' pmem namespace may be as capable as
today's 'memory mode' namespace.  Undo the encoding of the mode in the
device name and leave it to other tooling to determine the mode of the
namespace from its attributes.

Reported-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>