Replace ESRCH error with panic in procfs Readlink methods.
Replace ESRCH error with panic in procfs Readlink methods.

Use of RootFromContext(ctx) should always return a valid root in these cases.

The comment about "racing with process deletion" and ESRCH return value was
erroniously copy-pastad from the task.FSContext().RootDirectory() usage, where
such a race is possible.

PiperOrigin-RevId: 474885394

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor