nfsd: Fix nsfd startup race (again)

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

nfsd: Fix nsfd startup race (again)

Commit bd5ae9288d64 ("nfsd: register pernet ops last, unregister first")

has re-opened rpc_pipefs_event() race against nfsd_net_id registration

(register_pernet_subsys()) which has been fixed by commit bb7ffbf29e76

("nfsd: fix nsfd startup race triggering BUG_ON").

Restore the order of register_pernet_subsys() vs register_cld_notifier().

Add WARN_ON() to prevent a future regression.

Crash info:

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000012

CPU: 8 PID: 345 Comm: mount Not tainted 5.4.144-... #1

pc : rpc_pipefs_event+0x54/0x120 [nfsd]

lr : rpc_pipefs_event+0x48/0x120 [nfsd]

Call trace:

rpc_pipefs_event+0x54/0x120 [nfsd]

blocking_notifier_call_chain

rpc_fill_super

get_tree_keyed

rpc_fs_get_tree

vfs_get_tree

do_mount

ksys_mount

__arm64_sys_mount

el0_svc_handler

el0_svc