[IA64] perfmon: Change vmalloc to vzalloc and drop memset.

vzalloc() nicely zeroes memory for us, so we don't have to do a vmalloc()
and then manually memset() the returned memory when all we want is for it
to be zero. Patch changes this for pfm_rvmalloc().

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Tony Luck <tony.luck@intel.com>