linux/bitmap.h: remove redundant uses of small_const_nbits()

In the _zero, _fill and _copy functions, the small_const_nbits branch is
redundant.  If nbits is small and const, gcc knows full well that
BITS_TO_LONGS(nbits) is 1, so len is also a compile-time constant
(sizeof(long)), and calling memset or memcpy with a length argument of
sizeof(long) makes gcc generate the expected code anyway:

#include <string.h>
void a(unsigned long *x) { memset(x, 0, 8); }
void b(unsigned long *x) { memset(x, 0xff, 8); }
void c(unsigned long *x, const unsigned long *y) { memcpy(x, y, 8); }

turns into

0000000000000000 <a>:
   0:   48 c7 07 00 00 00 00    movq   $0x0,(%rdi)
   7:   c3                      retq
   8:   0f 1f 84 00 00 00 00    nopl   0x0(%rax,%rax,1)
   f:   00

0000000000000010 <b>:
  10:   48 c7 07 ff ff ff ff    movq   $0xffffffffffffffff,(%rdi)
  17:   c3                      retq
  18:   0f 1f 84 00 00 00 00    nopl   0x0(%rax,%rax,1)
  1f:   00

0000000000000020 <c>:
  20:   48 8b 06                mov    (%rsi),%rax
  23:   48 89 07                mov    %rax,(%rdi)
  26:   c3                      retq

Link: http://lkml.kernel.org/r/20180818131623.8755-4-linux@rasmusvillemoes.dk
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Yury Norov <ynorov@caviumnetworks.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>