make fls() and ffs() consistent across architectures

make fls() and ffs() consistent across architectures

Their parameter types differed between ARM and x86.

Along with generalizing the functions this fixes
- x86's non-long functions having long parameter types
- ARM's ffs() using a long intermediate variable
- generic_fls64() being broken when the upper half of the input is
  non-zero
- common (and in one case also ARM) code using fls() when flsl() was
  meant

Also drop ARM's constant_fls() in favor of the identical generic_fls().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>