byteswap: don't use __builtin_bswap*() with sparse

byteswap: don't use __builtin_bswap*() with sparse

Although sparse declares __builtin_bswap*(), it can't actually do
constant folding inside them (yet).  As such, things like

  switch (protocol) {
  case htons(ETH_P_IP):
          break;
  }

which we do all over the place cause sparse to warn that it expects a
constant instead of a function call.

Disable __HAVE_BUILTIN_BSWAP*__ if __CHECKER__ is defined to avoid this.

Fixes: e08d0bf1321a ("byteswap: try to avoid __builtin_constant_p gcc bug")
Link: http://lkml.kernel.org/r/1470914102-26389-1-git-send-email-johannes@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>