Without this, the default implementation is a no op which is completely
wrong with a VIVT cache, and usage of sg_copy_buffer() produces
unpredictable results.
Tested-by: NSebastian Andrzej Siewior <bigeasy@breakpoint.cc>

CC: stable@kernel.org
Signed-off-by: NNicolas Pitre <nico@marvell.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>