mkfs: add a config file for x86_64 pmem filesystems

mkfs: add a config file for x86_64 pmem filesystems

We have a handful of users who continually ping the maintainer with
questions about why pmem and dax don't work quite the way they want
(which is to say 2MB extents and PMD mappings) because they copy-pasted
some garbage from Google that's wrong.  Encode the correct defaults into
a mkfs config file and ship that.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>