Fix gcc array subscript above bounds warning

In a debug build, certain GCC versions flag an array bounds warning in
the below code from dnode_sync.c

    } else {
            int i;
            ASSERT(dn->dn_next_nblkptr[txgoff] < dnp->dn_nblkptr);
            /* the blkptrs we are losing better be unallocated */
            for (i = dn->dn_next_nblkptr[txgoff];
                i < dnp->dn_nblkptr; i++)
                    ASSERT(BP_IS_HOLE(&dnp->dn_blkptr[i]));

This usage is in fact safe, since the ASSERT ensures the index does
not exceed to maximum possible number of block pointers. However gcc
can't determine that the assignment 'i = dn->dn_next_nblkptr[txgoff];'
falls within the array bounds so it issues a warning.  To avoid this,
initialize i to zero to make gcc happy but skip the elements before
dn->dn_next_nblkptr[txgoff] in the loop body.  Since a dnode contains
at most 3 block pointers this overhead should be negligible.

Closes #950