nfsd: initialise nfsd_info.mutex early.
In the Linux kernel, the following vulnerability has been resolved:

nfsd: initialise nfsd_info.mutex early.

nfsd_info.mutex can be dereferenced by svc_pool_stats_start()
immediately after the new netns is created.  Currently this can
trigger an oops.

Move the initialisation earlier before it can possibly be dereferenced.