mtr_t::commit_file(): Protect the rename operation with fil_system.mutex
like we used to do before commit 2e43af69
in order to prevent fil_node_open_file() from running concurrently.
In other words, fil_system.mutex will protect the consistency of
fil_node_t::name and the file name in the file system.

This race condition should be very hard to trigger. We would need
a low value of innodb_open_files or table_cache limit so that
fil_space_t::try_to_close() will be invoked frequently. Simultaneously
with a RENAME operation, something (such as a write of a data page)
would have to try to open the file.