update ZFS on Linux

Fix #1957
* This patchset mainly addresses a race in ZFS, reported by a user [0].
* While adding this patch, I also updated the submodules to the latest
  versions, and dropped the patches we kept for 0.7.10 and 0.7.11
* Two other issues [1,2] are addressed, where 1 (a performance improvement)
  was a straight-forward cherry-pick and the other (a deadlock fix) needed
  minor backporting (mostly related to the SPL and ZFS repository merge with
  0.8)

[0] https://github.com/zfsonlinux/zfs/pull/8005
[1] https://github.com/zfsonlinux/zfs/pull/8011
[2] https://github.com/zfsonlinux/zfs/pull/7939

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

diff --git a/submodules/zfsonlinux b/submodules/zfsonlinux
index 0b063f2c200df2d28b2214b9590273568e0addca..851f3685395a7e2cc7a834e75a3f8c9c97e8dd13 160000 (submodule)
--- a/submodules/zfsonlinux
+++ b/submodules/zfsonlinux
@@ -1 +1 @@
-Subproject commit 0b063f2c200df2d28b2214b9590273568e0addca
+Subproject commit 851f3685395a7e2cc7a834e75a3f8c9c97e8dd13