mm: include swap.h in swapops.h

swapops.h uses MAX_SWAPFILES_SHIFT, SWP_MIGRATION_READ and SWP_PTE_MARKER,
all of which swap.h defines, but does not include swap.h.  It compiles only
where the translation unit pulled swap.h in first.  leafops.h includes
swapops.h on the line above swap.h, so a file whose include list reaches
leafops.h before swap.h gets:

    In file included from include/linux/leafops.h:11:
    include/linux/swapops.h:88:21: error: use of undeclared
    identifier 'MAX_SWAPFILES_SHIFT'

A header that uses a definition has to include the header that provides it.

Link: https://lore.kernel.org/20260818115026.656406-1-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202608181757.mza9RRj7-lkp@intel.com/
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Barry Song <baohua@kernel.org>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/swapops.h

@@ -5,6 +5,7 @@
 #include <linux/radix-tree.h>
 #include <linux/bug.h>
 #include <linux/mm_types.h>
+#include <linux/swap.h>
 
 #ifdef CONFIG_MMU