update ZFS to 2.1.6

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

diff --git a/src/submodules/zfsonlinux b/src/submodules/zfsonlinux
index af88d47f1ee86fae4abcfe0dabca67ea3c94d377..6a6bd493988c75331deab06e5352a9bed035a87d 160000 (submodule)
--- a/src/submodules/zfsonlinux
+++ b/src/submodules/zfsonlinux
@@ -1 +1 @@
-Subproject commit af88d47f1ee86fae4abcfe0dabca67ea3c94d377
+Subproject commit 6a6bd493988c75331deab06e5352a9bed035a87d