NFS: rename nfs_path variable

Clean up: I'm about to move the declaration of nfs_mount into
fs/nfs/internal.h and include it in fs/nfs/nfsroot.c.  There's a
conflicting definition of nfs_path in fs/nfs/internal.h and
fs/nfs/nfsroot.c, so rename the private one.

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