NFSD: Clean up kdoc for nfsd_open_local_fh()
Sparse reports that the synopsis of nfsd_open_local_fh() does not
match its kdoc comment. Introduced by commit e6f7e1487ab5
("nfs_localio: simplify interface to nfsd for getting nfsd_file").
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>