nfsd: fix NFSv4 time_delta attribute

Currently we return the worst-case value of 1 second in the time delta
attribute.  That's not terribly useful.  Instead, return a value
calculated from the time granularity supported by the filesystem and the
system clock.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>