copy_file_range() doesn't update the atime of an empty file

For a while, Jenkins has been complaining that one of the tmpfs tests is
failing:
https://ci.freebsd.org/job/FreeBSD-main-amd64-test/23814/testReport/junit/sys.fs.tmpfs/times_test/empty/

This has been happening since commit
8113cc827611a88540736c92ced7d3a7020a1723, which converted cat(1) to use
copy_file_range(2).  The test in question creates an empty file, waits
for a second, then cat(1)s it and checks that the file's atime was
updated.  After the aforementioned commit, the atime is not updated.

I believe the essential difference is that a zero-length read(2) results
in a call to VOP_READ(), which results in an updated atime even if no
bytes were read.  For instance, ffs_read() sets IN_ACCESS so long as the
routine doesn't return an error.  (I'm not sure if the mtime is
correspondingly updated upon a zero-length write.)

copy_file_range() on the other hand elides calls to VOP_READ/VOP_WRITE
when copylen is 0, so the atime doesn't get updated.  I wonder if we
could at least change it to call VOP_READ in that scenario, as in the
untested patch below.  Any thoughts?

diff --git a/sys/kern/vfs_vnops.c b/sys/kern/vfs_vnops.c
index 4e4161ef1a7f..d60608a6d3b9 100644
--- a/sys/kern/vfs_vnops.c
+++ b/sys/kern/vfs_vnops.c
@@ -3499,7 +3499,7 @@ vn_generic_copy_file_range(struct vnode *invp, off_t *inoffp,
 			xfer -= (*inoffp % blksize);
 		}
 		/* Loop copying the data block. */
-		while (copylen > 0 && error == 0 && !eof && interrupted == 0) {
+		while (error == 0 && !eof && interrupted == 0) {
 			if (copylen < xfer)
 				xfer = copylen;
 			error = vn_lock(invp, LK_SHARED);
@@ -3511,7 +3511,7 @@ vn_generic_copy_file_range(struct vnode *invp, off_t *inoffp,
 			    curthread);
 			VOP_UNLOCK(invp);
 			lastblock = false;
-			if (error == 0 && aresid > 0) {
+			if (error == 0 && (xfer == 0 || aresid > 0)) {
 				/* Stop the copy at EOF on the input file. */
 				xfer -= aresid;
 				eof = true;