update ZFS to 2.1.8

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

diff --git a/submodules/zfsonlinux b/submodules/zfsonlinux
index 3efcc79c39650c3d720414e957d566926119c4db..5e38018f531f899b5945eadf9e1838f818b0dea8 160000 (submodule)
--- a/submodules/zfsonlinux
+++ b/submodules/zfsonlinux
@@ -1 +1 @@
-Subproject commit 3efcc79c39650c3d720414e957d566926119c4db
+Subproject commit 5e38018f531f899b5945eadf9e1838f818b0dea8