This article describes the mentioned kernel error.

Sep 19 05:24:56 vsrx kernel: mlx5_core0: ERR: mlx5_init_once:942:(pid 6167): Unable to find vendor specific capabilities

Below messages are seen in the logs of SRX device:
Sep 19 05:24:56 vsrx kernel: mlx5_core0: ERR: mlx5_init_once:942:(pid 6167): Unable to find vendor specific capabilities
Sep 19 05:24:56 vsrx kernel: mlx5_core0: WARN: mlx5_fwdump_prep:76:(pid 6167): Failed to find vendor-specific capability, error 2
Sep 19 05:24:56 vsrx kernel: mlx5_core1: ERR: mlx5_init_once:942:(pid 6167): Unable to find vendor specific capabilities
Sep 19 05:24:56 vsrx kernel: mlx5_core1: WARN: mlx5_fwdump_prep:76:(pid 6167): Failed to find vendor-specific capability, error 2
Sep 19 05:24:56 vsrx kernel: if_sethwtsomax ** NOT IMPLEMENTED **if_sethwtsomaxsegcount ** NOT IMPLEMENTED **if_sethwtsomaxsegsize ** NOT IMPLEMENTED **mce0: ERR: mlx5e_create_ifp:4316:(pid 6167): Assigned random MAC address
Sep 19 05:24:56 vsrx kernel: if_sethwtsomax ** NOT IMPLEMENTED **if_sethwtsomaxsegcount ** NOT IMPLEMENTED **if_sethwtsomaxsegsize ** NOT IMPLEMENTED **mce1: ERR: mlx5e_create_ifp:4316:(pid 6167): Assigned random MAC address

These messages are seen since vSRX3.0 does not support diagnosis (an internal coded function), and do not cause any operational impact. 
This is not an error but an expected debug message.
Thus, these logs can be safely ignored. 

To stop seeing these messages, refer to KB9382 - How to prevent syslog messages from being written to the log file [juniper.net]  .