config-linux: Add restriction for duplicated device path

I think runtime should generate an error, if devices has
duplicated device path.
Because we don't know which one is really needed.

Signed-off-by: Ma Shimiao <mashimiao.fnst@cn.fujitsu.com>

Author: Ma Shimiao

config-linux.md

@@ -115,7 +115,9 @@ Each entry has the following structure:
 * **`type`** *(string, REQUIRED)* - type of device: `c`, `b`, `u` or `p`.
   More info in [mknod(1)][mknod.1].
 * **`path`** *(string, REQUIRED)* - full path to device inside container.
+  If a file already exists at `path` that does not match the requested device, the runtime MUST generate an error.
 * **`major, minor`** *(int64, REQUIRED unless **`type`** is `p`)* - [major, minor numbers][devices] for the device.
+  Same `major, minor` for different devices is acceptable, but it is not recommended.
 * **`fileMode`** *(uint32, OPTIONAL)* - file mode for the device.
   You can also control access to devices [with cgroups](#device-whitelist).
 * **`uid`** *(uint32, OPTIONAL)* - id of device owner.