Btrfs: make shrink_delalloc a little friendlier

Xfstests 224 will just sit there and spin for ever until eventually we give up
flushing delalloc and exit.  On my box this took several hours.  I could not
interrupt this process either, even though we use INTERRUPTIBLE.  So do 2 things

1) Keep us from looping over and over again without reclaiming anything
2) If we get interrupted exit the loop

I tested this and the test now exits in a reasonable amount of time, and can be
interrupted with ctrl+c.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>