update ZFS submodule to 0.7.9-3 with cherry-picked deadlock fix

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

diff --git a/submodules/zfsonlinux b/submodules/zfsonlinux
index cabb465d41a695d4db17729628e6694a645c083e..afa754176068d68c37e5abdaef27bac6837ceda2 160000 (submodule)
--- a/submodules/zfsonlinux
+++ b/submodules/zfsonlinux
@@ -1 +1 @@
-Subproject commit cabb465d41a695d4db17729628e6694a645c083e
+Subproject commit afa754176068d68c37e5abdaef27bac6837ceda2