copy_file_range: Call vn_rdwr() at least once

This ensures that we invoke VOP_READ on the input file even if it's
empty, which in turn helps ensure that filesystems update the atime of
the file.

PR: 274615
Reviewed by: olce, rmacklem, kib
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D43524