[SCSI] st: retry enlarge_buffer allocation

Make enlarge_buffer() retry allocation if the previously chosen page
order was too small. Really limit the page order to 6. Return error if
the maximum order is not large enough for the request.

Signed-off-by: Kai Makisara <Kai.Makisara@kolumbus.fi>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>