shmem: add large folios support to the write path

Add large folio support for shmem write path matching the same high
order preference mechanism used for iomap buffered IO path as used in
__filemap_get_folio().

Use the __folio_get_max_order to get a hint for the order of the folio
based on file size which takes care of the mapping requirements.

Swap does not support high order folios for now, so make it order 0 in
case swap is enabled.

Signed-off-by: Daniel Gomez <da.gomez@samsung.com>