fix and simplify futimesat fallback in utimensat

fix and simplify futimesat fallback in utimensat

previously the fallback wrongly failed with EINVAL rather than ENOSYS
when UTIME_NOW was used with one component but not both. commit
dd5f50da6f6c3df5647e922e47f8568a8896a752 introduced this behavior when
initially adding the fallback support.

instead, detect the case where both are UTIME_NOW early and replace
with a null times pointer; this may improve performance slightly (less
copy from user), and removes the complex logic from the fallback case.
it also makes things slightly simpler for adding time64 code paths.