nfsd: minor nfsd read api cleanup

Christoph points that the NFSv2/v3 callers know which case they want
here, so we may as well just call the file=NULL case directly instead of
making this conditional.

Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
index 835924f..f8931ac 100644
--- a/fs/nfsd/nfs4xdr.c
+++ b/fs/nfsd/nfs4xdr.c
@@ -2630,7 +2630,7 @@
 	}
 	read->rd_vlen = v;
 
-	nfserr = nfsd_read(read->rd_rqstp, read->rd_fhp, read->rd_filp,
+	nfserr = nfsd_read_file(read->rd_rqstp, read->rd_fhp, read->rd_filp,
 			read->rd_offset, resp->rqstp->rq_vec, read->rd_vlen,
 			&maxcount);