copy_file_range: Call vn_rdwr() even for zero-length files

[markj]

Summary

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

[rmacklem]

Looks ok to me.

[des]

Needs rebasing but passes the additional test from D44609.

[des]

sys/kern/vfs_vnops.c

3592

Instead of adding a flag, you could simply switch from while to do-while.

[markj]

sys/kern/vfs_vnops.c

3592

It's not that simple - we could have interrupted != 0 before the first iteration.