cp: Expect EINTR while copying

Both copy_file_range() and copy_fallback() can be interrupted before
they have read anything at all, in which case they return -1 and set
errno to EINTR.  If that happens, we should retry, not fail.

PR:		293028
MFC after:	1 week
Reviewed by:	kevans
Differential Revision:	https://reviews.freebsd.org/D55167

(cherry picked from commit 7aa30669d6)

bin/cp/utils.c

@@ -213,7 +213,10 @@ copy_file(const FTSENT *entp, int dne)
 		if (!use_copy_file_range) {
 			wcount = copy_fallback(from_fd, to_fd);
 		}
-		wtotal += wcount;
+		if (wcount >= 0)
+			wtotal += wcount;
+		else if (errno != EINTR)
+			break;
 		if (info) {
 			info = 0;
 			(void)fprintf(stderr,
@@ -221,7 +224,7 @@ copy_file(const FTSENT *entp, int dne)
 			    entp->fts_path, to.p_path,
 			    cp_pct(wtotal, fs->st_size));
 		}
-	} while (wcount > 0);
+	} while (wcount != 0);
 	if (wcount < 0) {
 		warn("%s", entp->fts_path);
 		rval = 1;