Revert "dpdk: drop dpdk submodule"

This reverts commit cb8087d.

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 ceph@cb8087d ?
<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 <[email protected]>

.gitmodules

@@ -40,6 +40,9 @@
 	url = https://github.com/ceph/lua.git
 	branch = lua-5.3-ceph
 	ignore = dirty
+[submodule "src/dpdk"]
+	path = src/dpdk
+	url = https://github.com/ceph/dpdk
 [submodule "src/zstd"]
 	path = src/zstd
 	url = https://github.com/facebook/zstd