zram regression fix backport

[Luis Henriques]

Hi,

It is known that commit 57ab048 ("zram: use zram->lock to protect
zram_free_page() in swap free notify path") has issues that are fixed
by commit a0c516c ("zram: don't grab mutex in zram_slot_free_noity").

Commit 57ab048 made its way to some of the stable kernels (3.2, 3.5,
3.8 and 3.10 I believe).  However, the backport of this fix (at least
to the older kernels) doesn't seem to be trivial -- to bring a0c516c
into these kernels, requires several other commits to be backported.

So, unless anyone has strong objections about it, I'm inclined to just
revert 57ab048 from the 3.5 kernel.

Cheers,
-- 
Luis