nfsd: Add quotes to client info 'callback address'

The 'callback address' in client_info_show is output without quotes
causing yaml parsers to fail on processing IPv6 addresses.
Adding quotes to 'callback address' also matches that used by
the 'address' field.

Signed-off-by: Mark Grimes <mark.grimes@ixsystems.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>