update ZFS to 2.1.5

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

diff --git a/submodules/zfsonlinux b/submodules/zfsonlinux
index 75a1232bf60e80a503a096884cd7dd641ca8f345..3f3541cad07018038b112fe1219895c210a30473 160000 (submodule)
--- a/submodules/zfsonlinux
+++ b/submodules/zfsonlinux
@@ -1 +1 @@
-Subproject commit 75a1232bf60e80a503a096884cd7dd641ca8f345
+Subproject commit 3f3541cad07018038b112fe1219895c210a30473