Seems to cause trouble with usb copying to external sd card. revert for now, will look at it later.

Revert "experimental page-writeback delayed until screen is off (accordingly to https://github.com/Firtecy/kowalski/commit/5c194eaf20bff3868bfe42f36a7b9e67c30518b9)"

This reverts commit 13d35493eee7ae532568fc43e524455a4155fa45.
Make block plug a configurable option and disable it by default for our android target.
Enable vm_highmem_is_dirtyable by default and prevent vm_dirty_bytes = 0 to also force vm_dirty_bytes = 0
tune page writeback a bit.
Made that old mmc blocks size hack a configurable option and enabled it by default
Make zram force page_cluster=0 when CONFIG_ANDROID is set, as per https://source.android.com/devices/low-ram.html recommendation.
Do actually implement word-at-a-time optimizations for fs/dcache.c
ditto for namei.c
fixes/workarounds/tunings for usb mass storage speed.
Just for consistent sake, revert this as it was superceeded by the more generic d12c11cc8d56af4da644a390d1d42e70252f0571

Revert "I/O plug weights too much on Android (which makes heavy use of Async I/O) compared to sdhci's internal buffering."

This reverts commit 6e9b02438b36ec45bf3ec90d95c63c9e934b7f68.
Updated default config in tegra_olympus_defconfig for what I'm using.