mm: swap: Change SWAP_NEXT_INVALID to highest value

We are about to introduce a scanning mechanism that can present 0 as a
valid cluster offset to scan_swap_map_try_ssd_cluster(), so let's change
SWAP_NEXT_INVALID to UINT_MAX, which is always invalid as an offset in
practice.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>