CONFIG_CPU_HAS_WB is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

Writeback Buffer available

modulename: wbflush.ko

configname: CONFIG_CPU_HAS_WB

Linux/MIPS Kernel Configuration
└─>Kernel type
└─>Writeback Buffer available
In linux kernel since version 2.6.12  
Say N here for slightly better performance. You must say Y here for
machines which require flushing of write buffers in software. Saying
Y is the safe option; N may result in kernel malfunction and crashes.

source code: