cifs: Remove struct reparse_posix_data from struct cifs_open_info_data

Linux SMB client already supports more reparse point types but only the
reparse_posix_data is defined in union of struct cifs_open_info_data.
This union is currently used as implicit casting between point types.

With this code style, it hides information that union is used for pointer
casting, and just in mknod_nfs() and posix_reparse_to_fattr() functions.

Other reparse point buffers do not use this kind of casting. So remove
reparse_posix_data from reparse part of struct cifs_open_info_data and for
all cases of reparse buffer use just struct reparse_data_buffer *buf.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>