Fix e7e136da (Add device_is_mounted() support for BSDs)

The actual C code was good, but the code to check getmntinfo(3)
in ./configure was missing * for st. getmntinfo(3) on FreeBSD/
DragonFlyBSD/etc takes statfs** as an argument.

I failed to catch this error as gcc without -Werror only gives
warning and sets $? to 0, though as a result it didn't break anything.

It did nothing to Linux since getmntinfo(3) doesn't compile on
Linux whether arg is * or **.

Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>