mm/zsmalloc: add __maybe_unused attribute for is_first_zpdesc()

Commit c1b3bb7 ("mm/zsmalloc: use zpdesc in
trylock_zspage()/lock_zspage()") introduces is_first_zpdesc() function. 
However, the function is only used when CONFIG_DEBUG_VM=y.

When building with LLVM=1 and W=1 option, the following warning is
generated:
  $ make -j12 W=1 LLVM=1 mm/zsmalloc.o
  mm/zsmalloc.c:455:20: error: function 'is_first_zpdesc' is not needed and will not be emitted [-Werror,-Wunneeded-internal-declaration]
    455 | static inline bool is_first_zpdesc(struct zpdesc *zpdesc)
        |                    ^~~~~~~~~~~~~~~
  1 error generated.

Fix the warning by adding __maybe_unused attribute to the function.
No functional change intended.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: c1b3bb7 ("mm/zsmalloc: use zpdesc in trylock_zspage()/lock_zspage()")
Signed-off-by: Hyeonggon Yoo <[email protected]>
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Cc: Alex Shi <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Sergey Senozhatsky <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: hygoni

mm/zsmalloc.c

@@ -452,7 +452,7 @@ static DEFINE_PER_CPU(struct mapping_area, zs_map_area) = {
 	.lock	= INIT_LOCAL_LOCK(lock),
 };
 
-static inline bool is_first_zpdesc(struct zpdesc *zpdesc)
+static inline bool __maybe_unused is_first_zpdesc(struct zpdesc *zpdesc)
 {
 	return PagePrivate(zpdesc_page(zpdesc));
 }