mm/mincore: use a helper for checking the swap cache

Introduce a mincore_swap helper for checking swap entries.  Move all swap
related logic and sanity debug check into it, and separate them from page
cache checking.

The performance is better after this commit.  mincore_page is never called
on a swap cache space now, so the logic can be simpler.  The sanity check
also covers more potential cases now, previously the WARN_ON only catches
potentially corrupted page table, now if shmem contains a swap entry with
!CONFIG_SWAP, a WARN will be triggered.  This changes the mincore value
when the WARN is triggered, but this shouldn't matter.  The WARN_ON means
the data is already corrupted or something is very wrong, so it really
should not happen.

Before this series:
mincore on a swaped out 16G anon mmap range:
Took 488220 us
mincore on 16G shmem mmap range:
Took 530272 us.

After this commit:
mincore on a swaped out 16G anon mmap range:
Took 446763 us
mincore on 16G shmem mmap range:
Took 460496 us.

About ~10% faster.

Link: https://lkml.kernel.org/r/20250811172018.48901-3-ryncsn@gmail.com


Signed-off-by: Kairui Song <kasong@tencent.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>