ANDROID: rust: uaccess: fix length usage in read_all

The `read_raw` call changes the value of `self.length` to zero, so the
last call to `set_len` did not actually change the length.

Android on cuttlefish will still boot even with this bug, which is why I
didn't discover it before now.

Bug: 326198426
Link: https://lore.kernel.org/all/CAH5fLgj--WrkLJLG69P59mxBjZvMqTMHjVCcbk3Q4z7ZFTeTRQ@mail.gmail.com/


Change-Id: I1ae8e1ee12ad81a99512767bee2b9093a19450a1
Signed-off-by: Alice Ryhl <aliceryhl@google.com>