iomap: Add __iomap_get_folio helper

Add an __iomap_get_folio() helper as the counterpart of the existing
__iomap_put_folio() helper.  Use the new helper in iomap_write_begin().
Not a functional change.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>