xdrgen: Generate "if" instead of "switch" for boolean union enumerators

xdrgen: Generate "if" instead of "switch" for boolean union enumerators

Eliminate this warning in code generated by xdrgen:

fs/nfsd/nfs3xdr_gen.c:220:2: warning: switch condition has boolean value [-Wswitch-bool]
  220 |         switch (ptr->attributes_follow) {
      |         ^       ~~~~~~~~~~~~~~~~~~~~~~

No more -Wswitch-bool warnings when compiling with W=1.

The generated code is functionally equivalent but somewhat more
idiomatic.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202511172336.Y75zj4v6-lkp@intel.com/
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>