netdevsim: Fix hwstats debugfs file permissions

The hwstats debugfs files are only writeable, but they are created with
read and write permissions, causing certain selftests to fail [1].

Fix by creating the files with write permission only.

[1]
 # ./test_offload.py
 Test destruction of generic XDP...
 Traceback (most recent call last):
   File "/home/idosch/code/linux/tools/testing/selftests/bpf/./test_offload.py", line 810, in <module>
     simdev = NetdevSimDev()
 [...]
 Exception: Command failed: cat /sys/kernel/debug/netdevsim/netdevsim0//ports/0/dev/hwstats/l3/disable_ifindex

 cat: /sys/kernel/debug/netdevsim/netdevsim0//ports/0/dev/hwstats/l3/disable_ifindex: Invalid argument

Fixes: 1a6d7ae7 ("netdevsim: Introduce support for L3 offload xstats")
Reported-by: Jie2x Zhou <jie2x.zhou@intel.com>
Tested-by: Jie2x Zhou <jie2x.zhou@intel.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20220909153830.3732504-1-idosch@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>