mm/memory-failure.c: remove unused macro `writeback'

mm/memory-failure.c: remove unused macro `writeback'

Unlike others we don't use the marco writeback.  so let's remove it to
tame gcc warning:

mm/memory-failure.c:827: warning: macro "writeback" is not used
[-Wunused-macros]

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Link: https://lkml.kernel.org/r/1599715096-20369-1-git-send-email-alex.shi@linux.alibaba.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>