nfs_common: convert int to bool

Since __state_in_grace return only true/false, make it return bool
instead of int.
Same change for the two user of it, locks_in_grace/opens_in_grace

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>