nfsd: don't ignore the return code of svc_proc_register()

In the Linux kernel, the following vulnerability has been resolved:

nfsd: don't ignore the return code of svc_proc_register()

Currently, nfsd_proc_stat_init() ignores the return value of

svc_proc_register(). If the procfile creation fails, then the kernel

will WARN when it tries to remove the entry later.

Fix nfsd_proc_stat_init() to return the same type of pointer as

svc_proc_register(), and fix up nfsd_net_init() to check that and fail

the nfsd_net construction if it occurs.

svc_proc_register() can fail if the dentry can't be allocated, or if an

identical dentry already exists. The second case is pretty unlikely in

the nfsd_net construction codepath, so if this happens, return -ENOMEM.