mm: filemap: retain mapped dropbehind folios

In the Linux kernel, the following vulnerability has been resolved:

mm: filemap: retain mapped dropbehind folios

Fault-around can map ready dropbehind folios without going through the
normal page-cache lookup that clears dropbehind. A mapping represents a
competing cached user, so retain the folio instead of forcibly unmapping
it when writeback completes.

For a mapped folio, folio_unmap_invalidate() can call
unmap_mapping_folio(), which takes i_mmap_rwsem and may sleep. Retaining
mapped folios avoids this path when folio_end_dropbehind() runs in
non-preemptible task context.

Tal was able to trigger a sleeping-in-atomic warning due to this [1].

Unmapped dropbehind folios continue through the existing invalidation path.