mm/swap_slots.c: assign|reset cache slot by value directly

Currently we use a tmp pointer, pentry, to transfer and reset swap cache
slot, which is a little redundant.  Swap cache slot stores the entry value
directly, assign and reset it by value would be straight forward.

Also this patch merges the else and if, since this is the only case we
refill and repeat swap cache.

Signed-off-by: Wei Yang <richard.weiyang@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Tim Chen <tim.c.chen@linux.intel.com>
Link: http://lkml.kernel.org/r/20200311055352.50574-1-richard.weiyang@linux.alibaba.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>