cifs: skip extra NULL byte in filenames

Since commit:
 cifs: alloc_path_with_tree_prefix: do not append sep. if the path is empty
alloc_path_with_tree_prefix() function was no longer including the
trailing separator when @path is empty, although @out_len was still
assuming a path separator thus adding an extra byte to the final
filename.

This has caused mount issues in some Synology servers due to the extra
NULL byte in filenames when sending SMB2_CREATE requests with
SMB2_FLAGS_DFS_OPERATIONS set.

Fix this by checking if @path is not empty and then add extra byte for
separator.  Also, do not include any trailing NULL bytes in filename
as MS-SMB2 requires it to be 8-byte aligned and not NULL terminated.

Cc: stable@vger.kernel.org
Fixes: 7eacba3b

 ("cifs: alloc_path_with_tree_prefix: do not append sep. if the path is empty")
Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>