gfs2: Check sb_bsize_shift after reading superblock

Fuzzers like to scribble over sb_bsize_shift but in reality it's very
unlikely that this field would be corrupted on its own. Nevertheless it
should be checked to avoid the possibility of messy mount errors due to
bad calculations. It's always a fixed value based on the block size so
we can just check that it's the expected value.

Tested with:

    mkfs.gfs2 -O -p lock_nolock /dev/vdb
    for i in 0 -1 64 65 32 33; do
        gfs2_edit -p sb field sb_bsize_shift $i /dev/vdb
        mount /dev/vdb /mnt/test && umount /mnt/test
    done

Before this patch we get a withdraw after

[   76.413681] gfs2: fsid=loop0.0: fatal: invalid metadata block
[   76.413681]   bh = 19 (type: exp=5, found=4)
[   76.413681]   function = gfs2_meta_buffer, file = fs/gfs2/meta_io.c, line = 492

and with UBSAN configured we also get complaints like

[   76.373395] UBSAN: shift-out-of-bounds in fs/gfs2/ops_fstype.c:295:19
[   76.373815] shift exponent 4294967287 is too large for 64-bit type 'long unsigned int'

After the patch, these complaints don't appear, mount fails immediately
and we get an explanation in dmesg.

Reported-by:  <syzbot+dcf33a7aae997956fe06@syzkaller.appspotmail.com>
Signed-off-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>