vn_copy_file_range(): use local variables for invp/outvp vnodes v_mounts
This avoids possible NULL dereference when checking mnt_vfc names.
Reviewed by: jah, rmacklem, Olivier Certner <olce.freebsd@certner.fr>
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D42554