INNODB_FAST_SHUTDOWN IS 2

Problem:

When innodb_fast_shutdown is set to 2 and the master thread enters
flush loop, under some circumstances it will not be able to exit it.
This may lead to a shutdown hanging.

This is happening because of the following:

1. In the flush_loop block of code, if the srv_fast_shutdown is
   equal to 2 (very fast shutdown), then we do not flush dirty
   pages in buffer pool to disk.
 
2. In the same flush_loop block of code, if the number of dirty
   pages is more than user specified limit, we go to step 1.

This results in infinite loop.

Solution:

When we are in the process of doing a very fast shutdown, don't
do step 2 above.

rb#2328 approved by Inaam.