blockdev-util: add new helper blockdev_get_device_size()

blockdev-util: add new helper blockdev_get_device_size()

This function is just a wrapper around the BLKGETSIZE64. Which is a
pretty simple ioctl. The only reason to wrap it, is that the headers we
need to call it are a bit messy (as "linux/fs.h" is incompatible with
certain glibc headers). Hence add the simple helper that wraps it and
allows us to do the header mess needed in one file only.

It's also nicely symmetric to blockdev_get_sector_size().