timespec_trunc() avoids rounding if granularity <= nanoseconds-per-jiffie
(or TICK_NSEC). This optimization assumes that:

 1. current_kernel_time().tv_nsec is already rounded to TICK_NSEC (i.e.
    with HZ=1000 you'd get 1000000, 2000000, 3000000... but never 1000001).
    This is no longer true (probably since hrtimers introduced in 2.6.16).

 2. TICK_NSEC is evenly divisible by all possible granularities. This may
    be true for HZ=100, 250, 1000, but obviously not for HZ=300 /
    TICK_NSEC=3333333 (introduced in 2.6.20).

Thus, sub-second portions of in-core file times are not rounded to on-disk
granularity. I.e. file times may change when the inode is re-read from disk
or when the file system is remounted.

This affects all file systems with file time granularities > 1 ns and < 1s,
e.g. CEPH (1000 ns), UDF (1000 ns), CIFS (100 ns), NTFS (100 ns) and FUSE
(configurable from user mode via struct fuse_init_out.time_gran).

Steps to reproduce with e.g. UDF:

  $ dd if=/dev/zero of=udfdisk count=10000 && mkudffs udfdisk
  $ mkdir udf && mount udfdisk udf
  $ touch udf/test && stat -c %y udf/test
  2015-06-09 10:22:56.130006767 +0200
  $ umount udf && mount udfdisk udf
  $ stat -c %y udf/test
  2015-06-09 10:22:56.130006000 +0200

Remounting truncates the mtime to 1 µs.

Fix the rounding in timespec_trunc() and update the documentation.

timespec_trunc() is exclusively used to calculate inode's [acm]time (mostly
via current_fs_time()), and always with super_block.s_time_gran as second
argument. So this can safely be changed without side effects.

Note: This does _not_ fix the issue for FAT's 2 second mtime resolution,
as super_block.s_time_gran isn't prepared to handle different ctime /
mtime / atime resolutions nor resolutions > 1 second.

Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: NKarsten Blees <blees@dcon.de>
Signed-off-by: NJohn Stultz <john.stultz@linaro.org>