fuse: reject internal errno

Don't allow userspace to report errors that could be kernel-internal.

Reported-by: Anatoly Trosinenko <anatoly.trosinenko@gmail.com>
Fixes: 334f485d

 ("[PATCH] FUSE - device functions")
Cc: <stable@vger.kernel.org> # v2.6.14
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>