Fix typo in commit b83e3e48c9b183a80dd00eb6c7431a1cbc7d89c9

There's a missing semicolon in the first hunk of this commit
in config/kernel-bdi.m4. This results in the test always failing.
The effects were noticed when rrdtool, a tool which modifies files
by mmap() and msync(), would have data never get saved to disk
in spite of the files working while the mounted filesystem remains
mounted.