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

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

Commit 069e1a20af92 ("swap_info: swap count continuations") introduced the
func add_swap_count_continuation() but forgot to use the helper function
swap_count() introduced by commit 0a5f43ac2756 ("mm: modify swap_map and
add SWAP_HAS_CACHE flag").

Link: https://lkml.kernel.org/r/20201009134306.18033-1-linmiaohe@huawei.com
Signed-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>