powerpc/tm: Fix null pointer deference in flush_hash_page

Make sure that current->thread.reg exists before we deference it in
flush_hash_page.
Signed-off-by: Michael Neuling <mikey@neuling.org>
Reported-by: John J Miller <millerjo@us.ibm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/mm/hash_utils_64.c

@@ -1230,6 +1230,7 @@ void flush_hash_page(unsigned long vpn, real_pte_t pte, int psize, int ssize,
 	 * unmapping it first, it may see the speculated version.
 	 */
 	if (local && cpu_has_feature(CPU_FTR_TM) &&
+	    current->thread.regs &&
 	    MSR_TM_ACTIVE(current->thread.regs->msr)) {
 		tm_enable();
 		tm_abort(TM_CAUSE_TLBI);