[SCSI] megaraid_sas: Fix synchronization problem between sysPD IO path and AEN path

There is syncronization problem between sysPD IO path and AEN path. Driver
maintains instance->pd_list[] array, which will get updated(by calling
function megasas_get_pd_list[]), whenever any of below events occurs-

MR_EVT_PD_INSERTED
MR_EVT_PD_REMOVED
MR_EVT_CTRL_HOST_BUS_SCAN_REQUESTED
MR_EVT_FOREIGN_CFG_IMPORTED

At same time running sysPD IO will be accessing the same array
instance->pd_list[], which is getting updated in AEN path, because of this IO
may not get correct PD info from instance->pd_list[] array.

Signed-off-by: Adam Radford <adam.radford@lsi.com>
Signed-off-by: Sumit Saxena <sumit.saxena@lsi.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>