mm/swapfile.c: use helper function swap_count() in add_swap_count_continuation()

Commit 570a335b ("swap_info: swap count continuations") introduced the
func add_swap_count_continuation() but forgot to use the helper function
swap_count() introduced by commit 355cfa73 ("mm: modify swap_map and
add SWAP_HAS_CACHE flag").

Link: https://lkml.kernel.org/r/20201009134306.18033-1-linmiaohe@huawei.comSigned-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/swapfile.c

@@ -3613,7 +3613,7 @@ int add_swap_count_continuation(swp_entry_t entry, gfp_t gfp_mask)
 
 	ci = lock_cluster(si, offset);
 
-	count = si->swap_map[offset] & ~SWAP_HAS_CACHE;
+	count = swap_count(si->swap_map[offset]);
 
 	if ((count & ~COUNT_CONTINUED) != SWAP_MAP_MAX) {
 		/*