Revert "zram: use zram->lock to protect zram_free_page() in swap free notify path"

This reverts commit 9e443904906ca2b5b3ae71f34ac4a4fa6905623e, which
was commit 57ab048532c0d975538cebd4456491b5c34248f4 upstream.

Taking the semaphore here leads to sleeping in atomic context.  This
was fixed in mainline commit a0c516cbfc74 ("zram: don't grab mutex in
zram_slot_free_noity") but that is too difficult to backport.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>