Avoid clobbering `errno` from `realpath` call in `__mount`

The __mount logic calls `realpath`, which can set errno in some cases (e.g. if a file is not found or the input string is malformed). The __mount algorithm here is attempting to use errno as signal of the `mount` syscall's success, which is not accurate if the value is clobbered.

This CL fixes the issue by clearing the errno explicitly just before the `mount` call. This ensures that any errno set is from the `mount` call itself. The existing `save_errno` field handles the logic for reading the previous invocation's errno value.

Bug: b/379929394
Change-Id: I840171079a71a1e7927fcc30d6c0863fab6c7e00