mm/zsmalloc.c: fix a -Wunused-function warning

set_zspage_inuse() was introduced in the commit 4f42047b ("zsmalloc:
use accessor") but all the users of it were removed later by the commits,

bdb0af7c ("zsmalloc: factor page chain functionality out")
3783689a ("zsmalloc: introduce zspage structure")

so the function can be safely removed now.

Link: http://lkml.kernel.org/r/1568658408-19374-1-git-send-email-cai@lca.pwSigned-off-by: Qian Cai <cai@lca.pw>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/zsmalloc.c

@@ -477,10 +477,6 @@ static inline int get_zspage_inuse(struct zspage *zspage)
 	return zspage->inuse;
 }
 
-static inline void set_zspage_inuse(struct zspage *zspage, int val)
-{
-	zspage->inuse = val;
-}
 
 static inline void mod_zspage_inuse(struct zspage *zspage, int val)
 {