cifs: fix crash querying symlinks stored as reparse-points

We never parsed/returned any data from .get_link() when the object is a windows reparse-point
containing a symlink. This results in the VFS layer oopsing accessing an uninitialized buffer:

...
[  171.407172] Call Trace:
[  171.408039]  readlink_copy+0x29/0x70
[  171.408872]  vfs_readlink+0xc1/0x1f0
[  171.409709]  ? readlink_copy+0x70/0x70
[  171.410565]  ? simple_attr_release+0x30/0x30
[  171.411446]  ? getname_flags+0x105/0x2a0
[  171.412231]  do_readlinkat+0x1b7/0x1e0
[  171.412938]  ? __ia32_compat_sys_newfstat+0x30/0x30
...

Fix this by adding code to handle these buffers and make sure we do return a valid buffer
to .get_link()

CC: Stable <stable@vger.kernel.org>
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>