MFV/ZoL: Fix stack dbuf_hold_impl()

MFV/ZoL:     Fix stack dbuf_hold_impl()

commit fc5bb51f08a6c91ff9ad3559d0266eeeab0b1f61
Author: Brian Behlendorf <behlendorf1@llnl.gov>
Date:   Thu Aug 26 10:52:00 2010 -0700

    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>