lib/bitmap.c: fix documentation for scnprintf() functions

The code comments for bscnl_emit() and bitmap_scnlistprintf() are
describing snprintf() return semantics, but these functions use
scnprintf() return semantics.  Fix that, and document the
bitmap_scnprintf() return value as well.

Cc: Ryota Ozaki <ozaki.ryota@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>