The v9fs_string_null() function just calls v9fs_string_free(). Also it
only has 4 users, whereas v9fs_string_free() has 87.

This patch converts users to call directly v9fs_string_free() and drops
the useless function.
Signed-off-by: NGreg Kurz <groug@kaod.org>
Reviewed-by: NCédric Le Goater <clg@kaod.org>