net: spider_net: Use non-atomic bitmap API when applicable

No concurrent access is possible when a bitmap is local to a function.
So prefer the non-atomic functions to save a few cycles.
   - replace a 'for' loop by an equivalent non-atomic 'bitmap_fill()' call
   - use '__set_bit()'

While at it, clear the 'bitmask' bitmap only when needed.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/3de0792f5088f00d135c835df6c19e63ae95f5d2.1638026251.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Jakub Kicinski <kuba@kernel.org>