basic: update fd_get_path() to use proc_mounted() helper

basic: update fd_get_path() to use proc_mounted() helper

We use it pretty much everywhere else, hence use it here too.

This also changes the error generated from EOPNOTSUPP to ENOSYS, to
match the other cases where we do such a check. One user checked for
EOPNOTSUPP which is updated to check for ENOSYS instead.