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.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Longlong Xia <[email protected]>
Reviewed-by: "Huang, Ying" <[email protected]>
Cc: Chen Wandun <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: Kefeng Wang <[email protected]>
Cc: Nanyong Sun <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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