fs: sysv: Fix sysv_nblocks() returns wrong value

sysv_nblocks() returns 'blocks' rather than 'res', which only counting
the number of triple-indirect blocks and causing sysv_getattr() gets a
wrong result.

[AV: this is actually a sysv counterpart of minixfs fix -
0fcd426de9d0 "[PATCH] minix block usage counting fix" in
historical tree; mea culpa, should've thought to check
fs/sysv back then...]

Fixes: 1da177e4

 ("Linux-2.6.12-rc2")
Signed-off-by: Chen Zhongjin <chenzhongjin@huawei.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>