OVL_FH_FID_OFFSET
BUILD_BUG_ON(OVL_FH_FID_OFFSET % 4);
BUILD_BUG_ON(MAX_HANDLE_SZ + OVL_FH_FID_OFFSET > 255);
fh = kzalloc(buflen + OVL_FH_FID_OFFSET, GFP_KERNEL);