mm/gup.c: mark undo_dev_pagemap as __maybe_unused

[ Upstream commit 790c73690c2bbecb3f6f8becbdb11ddc9bcff8cc ]

Several mips builds generate the following build warning.

  mm/gup.c:1788:13: warning: 'undo_dev_pagemap' defined but not used

The function is declared unconditionally but only called from behind
various ifdefs. Mark it __maybe_unused.

Link: http://lkml.kernel.org/r/1562072523-22311-1-git-send-email-linux@roeck-us.net
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

mm/gup.c

@@ -1367,7 +1367,8 @@ static inline pte_t gup_get_pte(pte_t *ptep)
 }
 #endif
 
-static void undo_dev_pagemap(int *nr, int nr_start, struct page **pages)
+static void __maybe_unused undo_dev_pagemap(int *nr, int nr_start,
+					    struct page **pages)
 {
 	while ((*nr) - nr_start) {
 		struct page *page = pages[--(*nr)];