POSIX mandates that a file's modification and change time be left
untouched upon truncate/ftruncate iff the file size does not change.
However, an open(O_TRUNC) call must always update the modification and
change time of the file, even if it was already zero-sized. VFS uses
the file systems' truncate call to implement O_TRUNC. This patch
replaces git-255ae85, which did not take into account the open case.
The size check is now moved into VFS, so that individual file systems
need not check for this case anymore.
POSIX truncate specification says "Upon successful completion, if
the *file size is changed*, this function shall mark for update the
st_ctime and st_mtime fields of the file." This patch prevents
changing of the date fields when the size stays the same.
