sbin/hammer: Make get_node() a wrapper over get_buffer_data()

sbin/hammer: Make get_node() a wrapper over get_buffer_data()

get_node() which is only used by hammer show/blockmap commands should
be a wrapper over the existing get_buffer_data().

It needs to do rel_buffer() if the given offset is out of range when
*bufferp is non NULL. Adding this check makes this function equivalent
of get_buffer_data() with 0 for isnew. isnew must be 0 because this
function assumes and only works when the B-Tree node already exists.

This doesn't change anything because hammer show/blockmap always give
NULL for *bufferp, but it should still be changed.