generic/172: raise fs size

generic/172: raise fs size

In original codes, if block size is 4096, fs size will be limited
to 60MB. In such small fs, btrfs is not able to write 40MB data,
because some space has been reserved internally, so the first
_pwrite_byte in this test case will fail for enospc error.

Also from this test case's test purpose, it tries to reflink a file
that uses more than half of the fs space, so here we can just define
fs size to be 256MB, the file to be reflinked to be 168MB.

Signed-off-by: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>