nfsd: cut the Linux NFSv4.1/4.2 some slack w.r.t. RFC5661

Recent testing of network partitioning a FreeBSD NFSv4.1
server from a Linux NFSv4.1 client identified problems
with both the FreeBSD server and Linux client.

Sometimes, after some Linux NFSv4.1/4.2 clients establish
a new TCP connection, they will advance the sequence number
for a session slot by 2 instead of 1.
RFC5661 specifies that a server should reply
NFS4ERR_SEQ_MISORDERED for this case.
This might result in a system call error in the client and
seems to disable future use of the slot by the client.
Since advancing the sequence number by 2 seems harmless,
allow this case if vfs.nfs.linuxseqsesshack is non-zero.

Note that, if the order of RPCs is actually reversed,
a subsequent RPC with a smaller sequence number value
for the slot will be received. This will result in
a NFS4ERR_SEQ_MISORDERED reply.
This has not been observed during testing.
Setting vfs.nfs.linuxseqsesshack to 0 will provide
RFC5661 compliant behaviour.

This fix affects the fairly rare case where a NFSv4
Linux client does a TCP reconnect and then apparently
erroneously increments the sequence number for the
session slot twice during the reconnect cycle.

PR: 254816
MFC after: 2 weeks