x86: Simplify flush_write_buffers()

Always make it an inline instead of using a macro for the no-op case.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
LKML-Reference: <1265380629-3212-7-git-send-email-brgerst@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>