swap: cleanup duplicated WARN_ON in add_to_avail_list

Patch series "fix WARN_ON in add_to_avail_list".


Empty check for plist_node is checked in add_to_avail_list and plist_add. 
Drop the duplicate one in add_to_avail_list.

Link: https://lkml.kernel.org/r/20230627120833.2230766-1-mawupeng1@huawei.com
Link: https://lkml.kernel.org/r/20230627120833.2230766-2-mawupeng1@huawei.com


Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>