ceph: initialize superblock s_time_gran to 1

Having granularity set to 1us results in having inode timestamps with a
accurancy different from the fuse client (i.e. atime, ctime and mtime will
always end with '000').  This patch normalizes this behaviour and sets the
granularity to 1.

Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Sage Weil <sage@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>