NFSv4.2: error out when relink swapfile

This fixes xfstests generic/356 failure on NFSv4.2.

Signed-off-by: Murphy Zhou <jencce.kernel@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/nfs4file.c

@@ -252,6 +252,9 @@ static loff_t nfs42_remap_file_range(struct file *src_file, loff_t src_off,
 	if (remap_flags & ~REMAP_FILE_ADVISORY)
 		return -EINVAL;
 
+	if (IS_SWAPFILE(dst_inode) || IS_SWAPFILE(src_inode))
+		return -ETXTBSY;
+
 	/* check alignment w.r.t. clone_blksize */
 	ret = -EINVAL;
 	if (bs) {