Return the bitmap size, rather than size + 1, when
bitmap_find_next_zero_area_off() cannot find a suitable area. This
matches the conventional find_bit() failure sentinel and still lets
callers detect failure with an out-of-range check.
Document the public failure contract as a value greater than or equal
to the bitmap size, without requiring callers to depend on the exact
sentinel.
Signed-off-by: Yury Norov <ynorov@nvidia.com>