ksmbd: use kasprintf() in ksmbd_vfs_xattr_stream_name()

Simplify the code by using kasprintf().  This also silences a Smatch
warning:

    fs/ksmbd/vfs.c:1725 ksmbd_vfs_xattr_stream_name()
    warn: inconsistent indenting

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>