ext4: add checksum calculation when clearing UNINIT flag in ext4_new_inode
When initializing an uninitialized block group in ext4_new_inode(),
its block group checksum must be re-calculated. This fixes a race
when several threads try to allocate a new inode in an UNINIT'd group.
There is some question whether we need to be initializing the block
bitmap in ext4_new_inode() at all, but for now, if we are going to
init the block group, let's eliminate the race.