scan_swap_map_slots() is only called by scan_swap_map() and
get_swap_pages().  Both ensure nr would not exceed SWAP_BATCH.

Just remove it.
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Hugh Dickins <hughd@google.com>
Link: http://lkml.kernel.org/r/20200325220309.9803-2-richard.weiyang@gmail.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>