mm: Don't hide spin_lock in swap_info_get

Now, swap_info_get hides lock holding by doing it internally
but releasing the lock is caller's duty. It's not serious bad
pattern but not good for readability, either.
More concern that if we uses swap_info_get in irq context,
the lock should be held with irq disabled.
So it would be better for caller to hold it because he can
judge the function will be used in irqcontext or not.

This patch will be used next patchset in this series.

Signed-off-by: Minchan Kim <minchan@kernel.org>