NFSv4: Fix OPEN / CLOSE race

NFSv4: Fix OPEN / CLOSE race

Ben Coddington has noted the following race between OPEN and CLOSE
on a single client.

Process 1		Process 2		Server
=========		=========		======

1)  OPEN file
2)			OPEN file
3)						Process OPEN (1) seqid=1
4)						Process OPEN (2) seqid=2
5)						Reply OPEN (2)
6)			Receive reply (2)
7)			new stateid, seqid=2

8)			CLOSE file, using
			stateid w/ seqid=2
9)						Reply OPEN (1)
10(						Process CLOSE (8)
11)						Reply CLOSE (8)
12)						Forget stateid
						file closed

13)			Receive reply (7)
14)			Forget stateid
			file closed.

15) Receive reply (1).
16) New stateid seqid=1
    is really the same
    stateid that was
    closed.

IOW: the reply to the first OPEN is delayed. Since "Process 2" does
not wait before closing the file, and it does not cache the closed
stateid, then when the delayed reply is finally received, it is treated
as setting up a new stateid by the client.

The fix is to ensure that the client processes the OPEN and CLOSE calls
in the same order in which the server processed them.

This commit ensures that we examine the seqid of the stateid
returned by OPEN. If it is a new stateid, we assume the seqid
must be equal to the value 1, and that each state transition
increments the seqid value by 1 (See RFC7530, Section 9.1.4.2,
and RFC5661, Section 8.2.2).

If the tracker sees that an OPEN returns with a seqid that is greater
than the cached seqid + 1, then it bumps a flag to ensure that the
caller waits for the RPCs carrying the missing seqids to complete.

Note that there can still be pathologies where the server crashes before
it can even send us the missing seqids. Since the OPEN call is still
holding a slot when it waits here, that could cause the recovery to
stall forever. To avoid that, we time out after a 5 second wait.

Reported-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>