vfs: fail only truncate on file descriptors opened read-only.

chmod is allowed.

Change-Id: Idcac38bdd7f0d614538421a41dfd30066a0c316f
Reviewed-on: https://review.haiku-os.org/c/1444
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>
Reviewed-by: Axel Dörfler <axeld@pinc-software.de>