[IA64] fix perfmon context load

The PFM_LOAD_CONTEXT may fail silently and cause a session
to remain reserved even though it should not. This can happen
when the commands succeeds in reserving the session but fails
when it actually tries to attach to the load_pid. In that case,
the command has failed but will return 0. More importantly,
the session will remain reserved. This patch fixes the problem.

Signed-off-by:  <stephane.eranian@hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>