NFSD: Fix the NFSv4.1 CREATE_SESSION operation

RFC 8881 Section 18.36.4 discusses the implementation of the NFSv4.1
CREATE_SESSION operation. The section defines four phases of
operation.

Phase 2 processes the CREATE_SESSION sequence ID. As a separate
step, Phase 3 evaluates the CREATE_SESSION arguments.

The problem we are concerned with is when phase 2 is successful but
phase 3 fails. The spec language in this case is "No changes are
made to any client records on the server."

RFC 8881 Section 18.35.4 defines a "client record", and it does
/not/ contain any details related to the special CREATE_SESSION
slot. Therefore NFSD is incorrect to skip incrementing the
CREATE_SESSION sequence id when phase 3 (see Section 18.36.4) of
CREATE_SESSION processing fails. In other words, even though NFSD
happens to store the cs_slot in a client record, in terms of the
protocol the slot is logically separate from the client record.

Three complications:

1. The world has moved on since commit 86c3e16c

 ("nfsd4: confirm
   only on succesful create_session") broke this. So we can't simply
   revert that commit.

2. NFSD's CREATE_SESSION implementation does not cleanly delineate
   the logic of phases 2 and 3. So this won't be a surgical fix.

3. Because of the way it currently handles the CREATE_SESSION slot
   sequence number, nfsd4_create_session() isn't caching error
   responses in the CREATE_SESSION slot. Instead of replaying the
   response cache in those cases, it's executing the transaction
   again.

Reorganize the CREATE_SESSION slot sequence number accounting. This
requires that error responses are appropriately cached in the
CREATE_SESSION slot (once it is found).

Reported-by: Connor Smith <connor.smith@hitachivantara.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218382
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>