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)