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

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

set_zspage_inuse() was introduced in the commit 4f42047bbde0 ("zsmalloc:
use accessor") but all the users of it were removed later by the commits,

bdb0af7ca8f0 ("zsmalloc: factor page chain functionality out")
3783689a1aa8 ("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.pw
Signed-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>