Require CAP_SEEK if both O_APPEND and O_TRUNC flags are absent.

In other words we don't require CAP_SEEK if either O_APPEND or O_TRUNC flag is
given, because O_APPEND doesn't allow to overwrite existing data and O_TRUNC
requires CAP_FTRUNCATE already.

Sponsored by:	The FreeBSD Foundation

sys/kern/vfs_syscalls.c

@@ -971,7 +971,7 @@ flags_to_rights(int flags)
 			/* FALLTHROUGH */
 		case O_WRONLY:
 			rights |= CAP_WRITE;
-			if (!(flags & O_APPEND))
+			if (!(flags & (O_APPEND | O_TRUNC)))
 				rights |= CAP_SEEK;
 			break;
 		}