In blr_open_binlog the refcnt increase of file which is already
open is protected by router->fileslock. In blr_close_binlog the
decrease of the refcnt was protected by file->lock.
This lead to a situation where it was possible that a file was
closed and the file instance freed, even though it just had been
taken into use by somebody else.
This is now fixed by solely using the router->fileslock for protecting
the increase and decrease of the refcnt.
e38334c457