swap: fold swap_ra_clamp_pfn into swap_ra_info

This makes the code cleaner.  This helper is made of only two line of self
explanational code and not reused anywhere else.

And this actually make the compiled object smaller by a bit.

bloat-o-meter results on x86_64 of mm/swap_state.o:

add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-35 (-35)
Function                                     old     new   delta
swap_ra_info.constprop                       512     477     -35
Total: Before=8388, After=8353, chg -0.42%

Link: https://lkml.kernel.org/r/20221219185840.25441-4-ryncsn@gmail.com


Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>