zonefs: Use str_plural() to fix Coccinelle warning

Fixes the following Coccinelle/coccicheck warning reported by
string_choices.cocci:

	opportunity for str_plural(zgroup->g_nr_zones)

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>