This patch doesn't get rid of the need to acquire the dict_sys->mutex but
reduces the need to keep the mutex locked for the duration of the query
to fsp_get_available_space_in_free_extents() from ha_innobase::info().
 
This is a port of revno:3548 from the builtin to the plugin.

rb://501 Approved by Jimmy Yang and Marko Makela.