generic/273: use _get_file_block_size

generic/273: use _get_file_block_size

This test calculates the amount of free space on a filesystem and uses
the block size to spread the work of filling the free space among a
bunch of threads.  Unfortunately, the test /should/ be using the
allocation unit size, not the fs block size, which is why this test
fails on configurations such as XFS realtime with a 28k extent size.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Leah Rumancik <leah.rumancik@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>