mm/filemap: Allow large folios to be added to the page cache

We return -EEXIST if there are any non-shadow entries in the page
cache in the range covered by the folio.  If there are multiple
shadow entries in the range, we set *shadowp to one of them (currently
the one at the highest index).  If that turns out to be the wrong
answer, we can implement something more complex.  This is mostly
modelled after the equivalent function in the shmem code.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>