fio: rename fdp.[c,h] to dataplacement.[c,h]

fio: rename fdp.[c,h] to dataplacement.[c,h]

We can use code in the files to support NVMe streams.  Streams also
falls under the umbrella of data placement, so it seems reasonable to
put streams and FDP code in the same source files.

Also change the prefix of some functions from fdp_ to dp_ to indicate
that they are not specific to FDP but apply more generally to the two
data placement features.

No functional change.

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>