NFS: Clean up nfs4_proc_get_rootfh

The long lines with no vertical white space make this function
difficult for humans to read.  Add a proper documenting comment
while we're here.

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