copy_file_range: Fix overlap checking

The check for range overlap did not correctly handle negative offests,
as the addition inoff + len is promoted to an unsigned type.

Reported by:	syzkaller
Reviewed by:	rmacklem
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D49674

sys/kern/vfs_syscalls.c

@@ -5075,6 +5075,15 @@ kern_copy_file_range(struct thread *td, int infd, off_t *inoffp, int outfd,
 	if (len == 0)
 		goto out;
 
+	/*
+	 * Make sure that the ranges we check and lock below are valid.  Note
+	 * that len is clamped to SSIZE_MAX above.
+	 */
+	if (inoff < 0 || outoff < 0) {
+		error = EINVAL;
+		goto out;
+	}
+
 	/*
 	 * If infp and outfp refer to the same file, the byte ranges cannot
 	 * overlap.