smb: server: handle readdir_info_level_struct_sz() error

early exit in smb2_populate_readdir_entry() if the requested info_level
is unknown.

Signed-off-by: Marios Makassikis <mmakassikis@freebox.fr>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>