[contrib][linux] Add zstd_common module

[terrelln]

The zstd_common module was added upstream in commit https://github.com/torvalds/linux/commit/637a642f5ca5e850186bb64ac75ebb0f124b458d.

But the kernel specific code was inlined into the library. This commit switches it to use the out of line method that we use for the other modules.