Revert "dpdk: drop dpdk submodule"

Revert "dpdk: drop dpdk submodule"

This reverts commit cb8087dfac31b8490fefdfca28d389b7b9901ef8.

The seastar submodule's .gitmodules links to `../dpdk` which is no longer present after removing dpdk from ceph.git's .gitmodules.

```
<dwfreed> the ceph/seastar repo uses awful URLs for the submodules
<dwfreed> and those awful URLs are the real reason it's failing
<dwfreed> dgalloway: ^^^
<dwfreed> seastar's .gitmodules references repos in the parent directory, so that when it's checked out as a submodule of ceph, you don't download the repos twice (and git will probably also use references instead of duplicating the local .git); however, ceph doesn't have a submodule for dpdk anymore
<dwfreed> so seastar's referencing a dpdk repo that doesn't exist
<dgalloway> i think i follow.  so you're suggesting revert https://github.com/ceph/ceph/commit/cb8087dfac31b8490fefdfca28d389b7b9901ef8 ?
<dwfreed> yep
<dwfreed> that'd be one way to fix it
...
<joshd> dgalloway: I'd suggest revert for now, and let the crimson folks figure out the longer term fix when they're back
```

Signed-off-by: David Galloway <dgallowa@redhat.com>