[IA64] another perfmon fix (take2)

- pfm_context_load(): change return value from EINVAL to EBUSY
  when context is already loaded.

- pfm_check_task_state(): pass test if context state is MASKED.
  It is safe to give access on PFM_CTX_MASKED because the PMU
  state (PMD) is stable and saved in software state.
  This helps multiplexing programs such as the example given
  in libpfm-3.1.

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