timekeeping: Add interfaces for handling timestamps with a floor value

Multigrain timestamps allow the kernel to use fine-grained timestamps when
an inode's attributes is being actively observed via ->getattr().  With
this support, it's possible for a file to get a fine-grained timestamp, and
another modified after it to get a coarse-grained stamp that is earlier
than the fine-grained time.  If this happens then the files can appear to
have been modified in reverse order, which breaks VFS ordering guarantees
[1].

To prevent this, maintain a floor value for multigrain timestamps.
Whenever a fine-grained timestamp is handed out, record it, and when later
coarse-grained stamps are handed out, ensure they are not earlier than that
value. If the coarse-grained timestamp is earlier than the fine-grained
floor, return the floor value instead.

Add a static singleton atomic64_t into timekeeper.c that is used to keep
track of the latest fine-grained time ever handed out. This is tracked as a
monotonic ktime_t value to ensure that it isn't affected by clock
jumps. Because it is updated at different times than the rest of the
timekeeper object, the floor value is managed independently of the
timekeeper via a cmpxchg() operation, and sits on its own cacheline.

Add two new public interfaces:

- ktime_get_coarse_real_ts64_mg() fills a timespec64 with the later of the
  coarse-grained clock and the floor time

- ktime_get_real_ts64_mg() gets the fine-grained clock value, and tries
  to swap it into the floor. A timespec64 is filled with the result.

The floor value is global and updated via a single try_cmpxchg(). If
that fails then the operation raced with a concurrent update. Any
concurrent update must be later than the existing floor value, so any
racing tasks can accept any resulting floor value without retrying.

[1]: POSIX requires that files be stamped with realtime clock values, and
     makes no provision for dealing with backward clock jumps. If a backward
     realtime clock jump occurs, then files can appear to have been modified
     in reverse order.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # documentation bits
Acked-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/all/20241002-mgtime-v10-1-d1c4717f5284@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

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