ext4: wait for log to commit when umounting

There is a potential race when a transaction is committing right when
the file system is being umounting.  This could reduce in a race
because EXT4_SB(sb)->s_group_info could be freed in ext4_put_super
before the commit code calls a callback so the mballoc code can
release freed blocks in the transaction, resulting in a panic trying
to access the freed s_group_info.

The fix is to wait for the transaction to finish committing before we
shutdown the multiblock allocator.  

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>