linux: Add fsopen

It was added on Linux 5.2 (24dcb3d90a1f67fe08c68a004af37df059d74005)
to start the process of preparing to create a superblock that will
then be mountable, using an fd as a context handle.

Tested-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>