mm/swapfile.c: remove unnecessary out label in __swap_duplicate()

mm/swapfile.c: remove unnecessary out label in __swap_duplicate()

When the code went to the out label, it must have p == NULL.  So what out
label really does is redundant if check and return err.  We should Remove
this unnecessary out label because it does not handle resource free and so
on.

Link: https://lkml.kernel.org/r/20201009130337.29698-1-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>