Fix undefined reference to `libzfs_fru_compare'

Add trivial libzfs_fru_compare() function which can be used when
HAVE_LIBTOPO is not defined. The only caller is find_vdev() and
this function should never be reached because search_fru must be
NULL unless HAVE_LIBTOPO is defined.

Rename _HAS_FMD_TOPO to existing HAVE_LIBTOPO which was
originally added for this purpose. This macro will never be defined.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5402
Closes #5909