utimens: avoid shadowing warning

utimens: avoid shadowing warning

lutimens declared struct stat st in two scopes.  Worse, on Linux
kernels between 2.6.18 and 2.6.22 (when utimensat existed, but
rejected AT_SYMLINK_NOFOLLOW) or before 2.6.18 (if the glibc
headers have utimensat, but the kernel does not), it would
result in redundant [fl]stat calls.

* lib/utimens.c (fdutimens, lutimens): Consolidate separate stat
buffers into one, to avoid shadowing, as well as avoiding a
redundant stat.
Reported by Jim Meyering.

Signed-off-by: Eric Blake <ebb9@byu.net>