lib/genalloc.c: change return type to unsigned long for bitmap_set_ll

Just as bitmap_clear_ll(), change return type to unsigned long
for bitmap_set_ll to avoid the possible overflow in future.

Link: https://lkml.kernel.org/r/20210105031644.2771-1-sjhuang@iluvatar.ai


Signed-off-by: Huang Shijie <sjhuang@iluvatar.ai>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>