sys-kernel: enable throttled background buffered writeback

This feature prevents the background writeback thread from stalling I/O,
e.g. reads.

For instance, without this option enabled, it's possible to stall reads
setting a writeback dirty threshold (/proc/sys/vm/dirty_writeback_centisecs)
sufficiently long.

Quoting the author of the feature:

"""
[..] If you are sick of Linux bogging down when buffered
writes are happening, then this is for you, laptop or server. The
patchset is fully stable, I have not observed problems. It passes full
xfstest runs, and a variety of benchmarks as well. It works equally well
on blk-mq/scsi-mq, and "classic" setups.
"""

"""
This feature adds a simple blk-wb code that keeps limits how much buffered
writeback we keep in flight on the device end. [..] it should be pretty
much auto-tuning.
"""

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>

sdk_container/src/third_party/coreos-overlay/sys-kernel/coreos-modules/files/commonconfig-4.14

@@ -47,6 +47,7 @@ CONFIG_MODULE_UNLOAD=y
 CONFIG_MODULE_SIG=y
 CONFIG_MODULE_SIG_SHA256=y
 CONFIG_BLK_DEV_THROTTLING=y
+CONFIG_BLK_WBT=y
 CONFIG_PARTITION_ADVANCED=y
 CONFIG_BSD_DISKLABEL=y
 CONFIG_MINIX_SUBPARTITION=y

sdk_container/src/third_party/coreos-overlay/sys-kernel/coreos-modules/files/commonconfig-4.15

@@ -47,6 +47,7 @@ CONFIG_MODULE_UNLOAD=y
 CONFIG_MODULE_SIG=y
 CONFIG_MODULE_SIG_SHA256=y
 CONFIG_BLK_DEV_THROTTLING=y
+CONFIG_BLK_WBT=y
 CONFIG_PARTITION_ADVANCED=y
 CONFIG_BSD_DISKLABEL=y
 CONFIG_MINIX_SUBPARTITION=y