NFS: Retry mounting NFSROOT

Lukas Razik <linux@razik.name> reports that on his SPARC system,
booting with an NFS root file system stopped working after commit
56463e50 "NFS: Use super.c for NFSROOT mount option parsing."

We found that the network switch to which Lukas' client was attached
was delaying access to the LAN after the client's NIC driver reported
that its link was up.  The delay was longer than the timeouts used in
the NFS client during mounting.

NFSROOT worked for Lukas before commit 56463e50 because in those
kernels, the client's first operation was an rpcbind request to
determine which port the NFS server was listening on.  When that
request failed after a long timeout, the client simply selected the
default NFS port (2049).  By that time the switch was allowing access
to the LAN, and the mount succeeded.

Neither of these client behaviors is desirable, so reverting 56463e50
is really not a choice.  Instead, introduce a mechanism that retries
the NFSROOT mount request several times.  This is the same tactic that
normal user space NFS mounts employ to overcome server and network
delays.

Signed-off-by: Lukas Razik <linux@razik.name>
[ cel: match kernel coding style, add proper patch description ]
[ cel: add exponential back-off ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: Lukas Razik <linux@razik.name>
Cc: stable@kernel.org # > 2.6.38
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>