fusefs: implement the "time_gran" feature.

fusefs: implement the "time_gran" feature.

If a server supports a timestamp granularity other than 1ns, it can tell the
client this as of protocol 7.23.  The client will use that granularity when
updating its cached timestamps during write.  This way the timestamps won't
appear to change following flush.

Sponsored by:	The FreeBSD Foundation