filemap: convert filemap_map_pmd() to take a folio

Patch series "Some more filemap folio conversions".

Three more places which could easily be converted to folios.  The third
one fixes a minor bug in readahead_expand(), but it's only a performance
bug and there are few users of readahead_expand(), so I don't think it's
worth backporting.


This patch (of 3):

Save a few calls to compound_head().  We specify exactly which page from
the folio to use by passing in start_pgoff, which means this will work for
a folio which is larger than PMD size.  The rest of the VM isn't prepared
for that yet, but now this function is.

Link: https://lkml.kernel.org/r/20230116193941.2148487-1-willy@infradead.org
Link: https://lkml.kernel.org/r/20230116193941.2148487-2-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>