ionic: fix 16bit math issue when PAGE_SIZE >= 64KB

The ionic device supports a maximum buffer length of 16 bits (see
ionic_rxq_desc or ionic_rxq_sg_elem).  When adding new buffers to
the receive rings, the function ionic_rx_fill() uses 16bit math when
calculating the number of pages to allocate for an RX descriptor,
given the interface's MTU setting. If the system PAGE_SIZE >= 64KB,
and the buf_info->page_offset is 0, the remain_len value will never
decrement from the original MTU value and the frag_len value will
always be 0, causing additional pages to be allocated as scatter-
gather elements unnecessarily.

A similar math issue exists in ionic_rx_frags(), but no failures
have been observed here since a 64KB page should not normally
require any scatter-gather elements at any legal Ethernet MTU size.

Fixes: 4b0a7539 ("ionic: implement Rx page reuse")
Signed-off-by: David Christensen <drc@linux.vnet.ibm.com>
Reviewed-by: Shannon Nelson <shannon.nelson@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>