When ipr's internal driver trace was changed to an atomic, a signed/unsigned
bug slipped in which results in us indexing backwards in our memory buffer
writing on memory that does not belong to us. This patch fixes this by removing
the modulo and instead just mask off the low bits.

Cc: <stable@vger.kernel.org>
Tested-by: NWen Xiong <wenxiong@linux.vnet.ibm.com>
Reviewed-by: NWen Xiong <wenxiong@linux.vnet.ibm.com>
Reviewed-by: NGabriel Krisman Bertazi <krisman@linux.vnet.ibm.com>
Signed-off-by: NBrian King <brking@linux.vnet.ibm.com>
Reviewed-by: NMartin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: NJames Bottomley <JBottomley@Odin.com>