ext4: fix invalid inode checksum

During the stability test, there are some errors:
  ext4_lookup:1590: inode #6967: comm fsstress: iget: checksum invalid.

If the inode->i_iblocks too big and doesn't set huge file flag, checksum
will not be recalculated when update the inode information to it's buffer.
If other inode marks the buffer dirty, then the inconsistent inode will
be flushed to disk.

Fix this problem by checking i_blocks in advance.

Cc: stable@kernel.org
Signed-off-by: Luo Meng <luomeng12@huawei.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Link: https://lore.kernel.org/r/20201020013631.3796673-1-luomeng12@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>