[IA64] Eliminate NULL test after alloc_bootmem in iosapic_alloc_rte()

As noted by Akinobu Mita alloc_bootmem and related functions never return
NULL and always return a zeroed region of memory.  Thus a NULL test or
memset after calls to these functions is unnecessary.

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Tony Luck <tony.luck@intel.com>