clean up __dbuf_hold_impl

clean up __dbuf_hold_impl

We can simplify the dbuf_hold code by allocating dbuf_hold_arg_t's on
demand, rather than allocating a big array of them up front.  While this
can occasionally increase the number of allocations, typically only one
allocation is needed since the indirect block is already cached.

The performance test suite gets the same results with this change.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #7841