iomap: push non-large folio check into get folio path

The len param to __iomap_get_folio() is primarily a folio allocation
hint. iomap_write_begin() already trims its local len variable based
on the provided folio, so move the large folio support check closer
to folio lookup.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Link: https://lore.kernel.org/20250506134118.911396-6-bfoster@redhat.com


Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>