sys-kernel: enable BFQ I/O scheduler

BFQ is a proportional-share storage-I/O scheduler for the
blk-mq layer. Its main features are:

 * Low latency for interactive applications
 * Low latency for soft real-time applications
 * Higher speed for code-development tasks
 * High throughput
 * Strong fairness, bandwidth and delay guarantees

Note that the default blk-mq scheduler is "None", so
th BFQ scheduler must be explicitly chosen for a given
storage device.

Note also that BFQ only supports blk-mq layer, so drivers
using legacy blk layer are not affected by this change.

In particular, both HDD and SSD storage drives use
the legacy blk layer by default.

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

sys-kernel/coreos-modules/files/commonconfig-4.14

@@ -53,6 +53,8 @@ CONFIG_MINIX_SUBPARTITION=y
 CONFIG_SOLARIS_X86_PARTITION=y
 CONFIG_UNIXWARE_DISKLABEL=y
 CONFIG_CFQ_GROUP_IOSCHED=y
+CONFIG_IOSCHED_BFQ=y
+CONFIG_BFQ_GROUP_IOSCHED=y
 CONFIG_PCIEPORTBUS=y
 CONFIG_HOTPLUG_PCI_PCIE=y
 CONFIG_PCIE_ECRC=y