NFSv4/flexfiles: support loosely coupled data servers

A flexfiles storage device is tightly coupled to the MDS only when the
decoded ds_versions[0].tightly_coupled flag is set (RFC 8435, sections
2.3 and 4.1). The client currently ignores that flag and treats every
data server as tightly coupled, which breaks I/O to loosely coupled DSes.

Two things force that assumption on an NFSv4.1+ DS:

  1) nfs4_set_ds_client() always sets NFS_CS_PNFS on the new client, so
     EXCHANGE_ID is sent with EXCHGID4_FLAG_USE_PNFS_DS.

  2) nfs4_init_ds_session() then calls is_ds_client() and returns -ENODEV
     if the reply does not carry EXCHGID4_FLAG_USE_PNFS_DS.

A loosely coupled DS is just a normal NFS server and does not act in the
pNFS DS role, so the client must not require it to advertise that role.

Thread the ds_versions[0].tightly_coupled flag from the flexfiles driver
down to the DS connect path. When it is false, skip both the NFS_CS_PNFS
flag and the is_ds_client() check. The file layout driver always passes
true because NFSv4.1 file layout data servers use the pNFS DS role.

Signed-off-by: Jeuk Kim <jeuk20.kim@samsung.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

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