nfsd4: tweak rd_dircount accounting

RFC 3530 14.2.24 says

This value represents the length of the names of the directory
entries and the cookie value for these entries. This length
represents the XDR encoding of the data (names and cookies)...

The "xdr encoding" of the name should probably include the 4 bytes for
the length.

But this is all just a hint so not worth e.g. backporting to stable.

Also reshuffle some lines to more clearly group together the
dircount-related code.

Signed-off-by: J. Bruce Fields <[e-mail]>

fs/nfsd/nfs4xdr.c

···
 	if (entry_bytes > cd->rd_maxcount)
 		goto fail;
 	cd->rd_maxcount -= entry_bytes;
-	if (!cd->rd_dircount)
-		goto fail;
 	/*
 	 * RFC 3530 14.2.24 describes rd_dircount as only a "hint", so
 	 * let's always let through the first entry, at least:
 	 */
-	name_and_cookie = 4 * XDR_QUADLEN(namlen) + 8;
+	if (!cd->rd_dircount)
+		goto fail;
+	name_and_cookie = 4 + 4 * XDR_QUADLEN(namlen) + 8;
 	if (name_and_cookie > cd->rd_dircount && cd->cookie_offset)
 		goto fail;
 	cd->rd_dircount -= min(cd->rd_dircount, name_and_cookie);
+
 	cd->cookie_offset = cookie_offset;
 skip_entry:
 	cd->common.err = nfs_ok;