device symlinks: just pass name, not struct driver_file_entry to driverfs_create_symlink

device symlinks: just pass name, not struct driver_file_entry to driverfs_create_symlink
symlinks now only use the name field of the struct driver_file_entry, so instead of allocating
a new one each time we want to create one, this changes the API to only accept the name (since
the driverfs core will never use the other fields either)

This also guarantees that we won't leak the memory for the entry when the device goes away:
Since the other entries aren't dynamically allocated, and driverfs doesn't free them, we would
have to free them ourselves. It's not really necessary, so this is another win for us.