Check for built-in tmpfs module

[mhorne]

Summary

As of r363471, tmpfs is included in all GENERIC kernel configs. The
tmpfs tests can still be run successfully, but emit a warning each time
they try to load tmpfs.ko:

module_register: cannot register tmpfs from tmpfs.ko; already loaded from kernel

Check for the presence of the module via kldstat first to quiet this
warning.

[ngie]

LGTM!