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/[e-mail]
Signed-off-by: Liu Shixin <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

fs/sysv/super.c

···
 	sbi->s_firstinodezone = 2;
 
 	flavour_setup[sbi->s_type](sbi, &sb->s_max_links);
-	
+	if (sbi->s_firstdatazone < sbi->s_firstinodezone)
+		return 0;
+
 	sbi->s_ndatazones = sbi->s_nzones - sbi->s_firstdatazone;
 	sbi->s_inodes_per_block = bsize >> 6;
 	sbi->s_inodes_per_block_1 = (bsize >> 6)-1;