cifs: Expand max share name length to 256

The old max share name length limit was 80 due to Windows NET SHARE
command not allowing more than that.  However, share names can be much
longer.  This is a more reasonable maximum share name length.

Signed-off-by: Scott Lovenberg <scott.lovenberg@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>