LU-11986 lnet: Avoid lnet debugfs read/write if ctl_table does not exist

LU-11986 lnet: Avoid lnet debugfs read/write if ctl_table does not exist

Running command "lctl get param -n stats" after lnet
is taken down leads to kernel panic because it
tries to read from the file which doesnt exist
anymore.

In lnet_debugfs_read() and lnet_debugfs_write(),
check if struct ctl_table is valid before trying
to read/write to it.

Change-Id: I2450d2f89c2e8a7db793680a4df581282ee46a16
Signed-off-by: Sonia Sharma <sharmaso@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/34634
Tested-by: Jenkins
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>