Fix truncate(2) mtime and ctime handling

On Linux, ftruncate(2) always changes the file timestamps, even if the
file size is not changed. However, in case of a successfull
truncate(2), the timestamps are updated only if the file size changes.
This translates to the VFS calling the ZFS Posix Layer "setattr"
function (zpl_setattr) with ATTR_MTIME and ATTR_CTIME unconditionally
set on the iattr mask only when doing a ftruncate(2), while the
truncate(2) is left to the filesystem implementation to be dealt with.

This behaviour is consistent with POSIX:2004/SUSv3 specifications
where there's no explicit requirement for file size changes to update
the timestamps only for ftruncate(2):

http://pubs.opengroup.org/onlinepubs/009695399/functions/truncate.html
http://pubs.opengroup.org/onlinepubs/009695399/functions/ftruncate.html

This has been later updated in POSIX:2008/SUSv4 where, for both
truncate(2)/ftruncate(2), there's no mention of this size change
requirement:

http://austingroupbugs.net/view.php?id=489
http://pubs.opengroup.org/onlinepubs/9699919799/functions/truncate.html
http://pubs.opengroup.org/onlinepubs/9699919799/functions/ftruncate.html

Unfortunately the Linux VFS is still calling into the ZPL without
ATTR_MTIME/ATTR_CTIME set in the truncate(2) case: we fix this by
explicitly updating the timestamps when detecting the ATTR_SIZE bit,
which is always set in do_truncate(), on the iattr mask.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6811
Closes #6819