nfs_common: Synchronize access to the SSC client ops table

nfsd42_ssc_open() and nfsd42_ssc_close() load ssc_nfs4_ops without
synchronization while nfs42_ssc_register() and nfs42_ssc_unregister()
store to it. Those reads are safe today only through a non-obvious
invariant: an inter-server copy holds an active vers=4.2 mount of the
source across both calls, the mount pins the nfsv4 module through the
nfs_client's cl_nfs_mod reference, and unregister runs only at nfsv4
module exit, so it cannot run while a call is in flight. Replace that
implicit contract with synchronization local to the broker, so its
safety no longer rests on a caller in another subsystem.

Read the pointer under RCU so a reader observes it atomically as a
valid table or NULL. nfs42_ssc_unregister() stores NULL and then calls
synchronize_rcu(), so it cannot return while a reader still holds the
pointer.

The two readers need different handling because one sleeps and the
other does not. sco_close() does not sleep, so nfsd42_ssc_close() runs
it to completion inside the RCU read-side section and the
synchronize_rcu() in unregister waits for it. __nfs42_ssc_open() does
sleep -- it issues a GETATTR RPC to the source server and allocates
with GFP_KERNEL -- so it must not run inside an RCU read-side section.
Pin the provider module with try_module_get() while still under
rcu_read_lock(), drop the lock, invoke the open, then release the
module. The reference keeps the provider mapped across the sleep
without relying on the caller's mount. If the table has already been
torn down the copy gets -EIO.

Cc: Olga Kornievskaia <okorniev@redhat.com>
Cc: Dai Ngo <dai.ngo@oracle.com>
Link: https://patch.msgid.link/20260721162306.894558-4-cel@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>

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