ibcore: Protect against concurrent access to hardware stats.

Currently access to hardware stats buffer isn't protected, this can
result in multiple writes and reads at the same time to the same
memory location. This can lead to providing an incorrect value to
the user. Add a mutex to protect against it.

Linux commit:
e945130b52bea65d15f9bdf54949d4cb7a88db7f

MFC after: 1 week
Reviewed by: kib
Sponsored by: Mellanox Technologies // NVIDIA Networking