Fix stack dbuf_hold_impl()

This commit preserves the recursive function dbuf_hold_impl() but moves
the local variables and function arguments to the heap to minimize
the stack frame size.  Enough space is initially allocated on the
stack for 20 levels of recursion.  This technique was based on commit
34229a2f2ac07363f64ddd63e014964fff2f0671 which reduced stack usage of
traverse_visitbp().

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>