ext4: check mount's handling for very large s_first_meta_bg

On ext4 filesystem, the kernel carshes at mount time when
s_first_meta_bg's value exceeds the largest possible meta_bg
number.  This kernel bug has been fixed in:

3a4b77c ext4: validate s_first_meta_bg at mount time

[eguan: add comments on the first_meta_bg value]

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>
3 files changed