FROMLIST: fuse: Fix crediantials leak in passthrough read_iter

If the system doesn't have enough memory when fuse_passthrough_read_iter
is requested in asynchronous IO, an error is directly returned without
restoring the caller's credentials.
Fix by always ensuring credentials are restored.

Fixes: aa29f32988c1f84c96e2457b049dea437601f2cc ("FROMLIST: fuse: Use daemon creds in passthrough mode")
Link: https://lore.kernel.org/lkml/YB0qPHVORq7bJy6G@google.com/


Reported-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Alessio Balsini <balsini@android.com>
Signed-off-by: Alessio Balsini <balsini@google.com>
Change-Id: I4aff43f5dd8ddab2cc8871cd9f81438963ead5b6