vsprintf: rework bitmap_list_string

bitmap_list_string() is very ineffective when printing bitmaps with long
ranges of set bits because it calls find_next_bit for each bit in the
bitmap.  We can do better by detecting ranges of set bits.

In my environment, before/after is 943008/31008 ns.

Signed-off-by: Yury Norov <yury.norov@gmail.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>