update ZFS to 0.7.4 + ARC hit rate cherry-pick

from 0.7.6 queue

diff --git a/submodules/zfsonlinux b/submodules/zfsonlinux
index b03175e961a650c10e84bbff12657881acf32b4c..fdd7076d71bc518d425dc2266982be6df5ede486 160000 (submodule)
--- a/submodules/zfsonlinux
+++ b/submodules/zfsonlinux
@@ -1 +1 @@
-Subproject commit b03175e961a650c10e84bbff12657881acf32b4c
+Subproject commit fdd7076d71bc518d425dc2266982be6df5ede486