mount 'mellanox' folder only instead of create each sub folder (#7830)

#### Why I did it

Following the discussion in another PR https://github.com/Azure/sonic-buildimage/pull/7708#discussion_r642933510 , since there will be multi subfolders under **/var/log/mellanox**, so we agreed to only mount this folder and the subfolders will be created afterward on demand.  

#### How I did it

during the syncd docker creation, only mount  folder **/var/log/mellanox**

#### How to verify it

build an Mellanox image and verify the related folder on the host and docker side.

files/build_templates/docker_image_ctl.j2

@@ -354,7 +354,7 @@ start() {
 {%- endif %}
 {%- if sonic_asic_platform == "mellanox" %}
 {%- if docker_container_name == "syncd" %}
-        -v /var/log/mellanox/sniffer:/var/log/mellanox/sniffer:rw \
+        -v /var/log/mellanox:/var/log/mellanox:rw \
         -v mlnx_sdk_socket:/var/run/sx_sdk \
         -v mlnx_sdk_ready:/tmp \
         -v /dev/shm:/dev/shm:rw \