mm/filemap: fix storing to a THP shadow entry

When a THP is removed from the page cache by reclaim, we replace it with a
shadow entry that occupies all slots of the XArray previously occupied by
the THP.  If the user then accesses that page again, we only allocate a
single page, but storing it into the shadow entry replaces all entries
with that one page.  That leads to bugs like

page dumped because: VM_BUG_ON_PAGE(page_to_pgoff(page) != offset)
------------[ cut here ]------------
kernel BUG at mm/filemap.c:2529!

https://bugzilla.kernel.org/show_bug.cgi?id=206569

This is hard to reproduce with mainline, but happens regularly with the
THP patchset (as so many more THPs are created).  This solution is take
from the THP patchset.  It splits the shadow entry into order-0 pieces at
the time that we bring a new page into cache.

Fixes: 99cb0dbd

 ("mm,thp: add read-only THP support for (non-shmem) FS")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Qian Cai <cai@lca.pw>
Link: https://lkml.kernel.org/r/20200903183029.14930-4-willy@infradead.org


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>