src/t_stripealign.c: Use block size reported by fstatfs()

src/t_stripealign.c: Use block size reported by fstatfs()

On ppc64le, block size reported by fstat(2) is 64k (the page size)
i.e. the "preferred I/O size". However src/t_stripealign.c requires the
actual block size of the filesystem. Hence this commit now makes use of
the block size reported by fstatfs(2) syscall.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>