xtensa: provide __NR_sync_file_range2 instead of __NR_sync_file_range

xtensa: provide __NR_sync_file_range2 instead of __NR_sync_file_range

xtensa actually uses sync_file_range2 implementation, so it should
define __NR_sync_file_range2 as other architectures that use that
function. That fixes userspace interface (that apparently never worked)
and avoids special-casing xtensa in libc implementations.
See the thread ending at
http://lists.busybox.net/pipermail/uclibc/2015-February/048833.html
for more details.

Cc: stable@vger.kernel.org
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>