Impact: reduce text size

bitmap_zero et al have a fastpath for nbits <= BITS_PER_LONG, but this
should really only apply where the nbits is known at compile time.

This only saves about 1200 bytes on an allyesconfig kernel, but with
cpumasks going variable that number will increase.

   text		data	bss	dec		hex	filename
35327852        5035607 6782976 47146435        2cf65c3 vmlinux-before
35326640        5035607 6782976 47145223        2cf6107 vmlinux-after

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>