Fix harmless "BARRIER is deprecated" kernel warning on Centos 6.8
A one time warning after module load that "BARRIER is deprecated" was seen
on the heavily patched 2.6.32-642.13.1.el6.x86_64 Centos 6.8 kernel. It seems
that kernel had both the old BARRIER and the newer FLUSH/FUA interfaces
defined. This fixes the warning by prefering the newer FLUSH/FUA interface
if it's available.
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #5739
Closes #5828