dm log writes: add support for inline data buffers

Currently dm-log-writes supports writing filesystem data via BIOs, and
writing internal metadata from a flat buffer via write_metadata().

For DAX writes, though, we won't have a BIO, but will instead have an
iterator that we'll want to use to fill a flat data buffer.

So, create write_inline_data() which allows us to write filesystem data
using a flat buffer as a source, and wire it up in log_one_block().

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>