mm/swapfile.c: fix comments for swapcache_prepare

The -EEXIST returned by __swap_duplicate means there is a swap cache
instead -EBUSY

Signed-off-by: Chen Wandun <chenwandun@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20200212145754.27123-1-chenwandun@huawei.com


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