bitmap: relax find_nth_bit() limitation on return value

The function claims to return the bitmap size, if Nth bit doesn't exist.
This rule is violated in inline case because the fns() that is used
there doesn't know anything about size of the bitmap.

So, relax this requirement to '>= size', and make the outline
implementation a bit cheaper.

All in-tree kernel users of find_nth_bit() are safe against that.

Reported-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Closes: https://lore.kernel.org/all/Zi50cAgR8nZvgLa3@yury-ThinkPad/T/#m6da806a0525e74dcc91f35e5f20766ed4e853e8a


Signed-off-by: Yury Norov <yury.norov@gmail.com>