NFS: Avoid PUTROOTFH when managing leases

Currently, the compound operation the Linux NFS client sends to the
server to confirm a client ID looks like this:

	{ SETCLIENTID_CONFIRM; PUTROOTFH; GETATTR(lease_time) }

Once the lease is confirmed, it makes sense to know how long before
the client will have to renew it.  And, performing these operations
in the same compound saves a round trip.

Unfortunately, this arrangement assumes that the security flavor
used for establishing a client ID can also be used to access the
server's pseudo-fs.

If the server requires a different security flavor to access its
pseudo-fs than it allowed for the client's SETCLIENTID operation,
the PUTROOTFH in this compound fails with NFS4ERR_WRONGSEC.  Even
though the SETCLIENTID_CONFIRM succeeded, our client's trunking
detection logic interprets the failure of the compound as a failure
by the server to confirm the client ID.

As part of server trunking detection, the client then begins another
SETCLIENTID pass with the same nfs4_client_id.  This fails with
NFS4ERR_CLID_INUSE because the first SETCLIENTID/SETCLIENTID_CONFIRM
already succeeded in confirming that client ID -- it was the
PUTROOTFH operation that caused the SETCLIENTID_CONFIRM compound to
fail.

To address this issue, separate the "establish client ID" step from
the "accessing the server's pseudo-fs root" step.  The first access
of the server's pseudo-fs may require retrying the PUTROOTFH
operation with different security flavors.  This access is done in
nfs4_proc_get_rootfh().

That leaves the matter of how to retrieve the server's lease time.
nfs4_proc_fsinfo() already retrieves the lease time value, though
none of its callers do anything with the retrieved value (nor do
they mark the lease as "renewed").

Note that NFSv4.1 state recovery invokes nfs4_proc_get_lease_time()
using the lease management security flavor.  This may cause some
heartburn if that security flavor isn't the same as the security
flavor the server requires for accessing the pseudo-fs.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>