exfat: use iter_file_splice_write

Doing copy_file_range() on exfat with a file opened for direct IO leads
to an -EFAULT:

# xfs_io -f -d -c "truncate 32768" \
       -c "copy_range -d 16384 -l 16384 -f 0" /mnt/test/junk
copy_range: Bad address

and the reason seems to be that we go through:

default_file_splice_write
 splice_from_pipe
  __splice_from_pipe
   write_pipe_buf
    __kernel_write
     new_sync_write
      generic_file_write_iter
       generic_file_direct_write
        exfat_direct_IO
         do_blockdev_direct_IO
          iov_iter_get_pages

and land in iterate_all_kinds(), which does "return -EFAULT" for our kvec
iter.

Setting exfat's splice_write to iter_file_splice_write fixes this and lets
fsx (which originally detected the problem) run to success from
the xfstests harness.
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>

fs/exfat/file.c

@@ -348,12 +348,13 @@ int exfat_setattr(struct dentry *dentry, struct iattr *attr)
 }
 
 const struct file_operations exfat_file_operations = {
-	.llseek      = generic_file_llseek,
-	.read_iter   = generic_file_read_iter,
-	.write_iter  = generic_file_write_iter,
-	.mmap        = generic_file_mmap,
-	.fsync       = generic_file_fsync,
-	.splice_read = generic_file_splice_read,
+	.llseek		= generic_file_llseek,
+	.read_iter	= generic_file_read_iter,
+	.write_iter	= generic_file_write_iter,
+	.mmap		= generic_file_mmap,
+	.fsync		= generic_file_fsync,
+	.splice_read	= generic_file_splice_read,
+	.splice_write	= iter_file_splice_write,
 };
 
 const struct inode_operations exfat_file_inode_operations = {