mm/swapfile: add cond_resched() in get_swap_pages()

In the Linux kernel, the following vulnerability has been resolved:

mm/swapfile: add cond_resched() in get_swap_pages()

The softlockup still occurs in get_swap_pages() under memory pressure. 64

CPU cores, 64GB memory, and 28 zram devices, the disksize of each zram

device is 50MB with same priority as si. Use the stress-ng tool to

increase memory pressure, causing the system to oom frequently.

The plist_for_each_entry_safe() loops in get_swap_pages() could reach tens

of thousands of times to find available space (extreme case:

cond_resched() is not called in scan_swap_map_slots()). Let's add

cond_resched() into get_swap_pages() when failed to find available space

to avoid softlockup.