fs: sysv: check sbi->s_firstdatazone in complete_read_super

sbi->s_firstinodezone is initialized to 2 and sbi->s_firstdatazone is read
from sbd.  There's no guarantee that sbi->s_firstdatazone must bigger than
sbi->s_firstinodezone.  If sbi->s_firstdatazone less than 2, the
filesystem can still be mounted unexpetly.  At this point, sbi->s_ninodes
flip to very large value and this filesystem is broken.  We can observe
this by executing 'df' command.  When we execute, we will get an error
message:

	"sysv_count_free_inodes: unable to read inode table"

Link: https://lkml.kernel.org/r/20220330104215.530223-1-liushixin2@huawei.com


Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>