btrfs: fix deadlock with concurrent chunk allocations involving system chunks

      When a task attempting to allocate a new chunk verifies that there is not
currently enough free space in the system space_info and there is another
task that allocated a new system chunk but it did not finish yet the
creation of the respective block group, it waits for that other task to
finish creating the block group. This is to avoid exhaustion of the system
chunk array in the superblock, which is limited, when we have a thundering
herd of tasks allocating new chunks. This problem was described and fixed
by commit eafa4fd0ad0607 ("btrfs: fix exhaustion of the system chunk array
due to concurrent allocations").

However there are two very similar scenarios where this can lead to a
deadlock:

1) Task B allocated a new system chunk and task A is waiting on task B
   to finish creation of the respective system block group. However before
   task B ends its transaction handle and finishes the creation of the