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.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory-failure.c

@@ -825,7 +825,6 @@ static int me_huge_page(struct page *p, unsigned long pfn)
 #define sc		((1UL << PG_swapcache) | (1UL << PG_swapbacked))
 #define unevict		(1UL << PG_unevictable)
 #define mlock		(1UL << PG_mlocked)
-#define writeback	(1UL << PG_writeback)
 #define lru		(1UL << PG_lru)
 #define head		(1UL << PG_head)
 #define slab		(1UL << PG_slab)
@@ -874,7 +873,6 @@ static struct page_state {
 #undef sc
 #undef unevict
 #undef mlock
-#undef writeback
 #undef lru
 #undef head
 #undef slab