proc_fuse: improve swap calculation

proc_fuse: improve swap calculation

Stéphane has been running into issues with wrong sawp limits. The logic
he suggested does:
- if memory == memory+swap, we list both at the memlimit for total size
- if memory+swap is larger than memory, then we add the extra amount to
  total swap but still cap to physical swap limit

We came to the conclusion that this is currently the best approach as we
show the mem limit as both Memory and Swap so advertising twice as much
as allowed but there's no way around that really or we'd need to
dynamically update both MemTotal and SwapTotal based on what's used to
keep MemTotal+SwapTotal == limit that's certainly doable but it will be
somewhat racy and also likely confused userspace more than mis-reporting
swap.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>