hexagon: fix livelock in uaccess

hexagon equivalent of 26178ec1 "x86: mm: consolidate VM_FAULT_RETRY handling"
If e.g. get_user() triggers a page fault and a fatal signal is caught, we might
end up with handle_mm_fault() returning VM_FAULT_RETRY and not doing anything
to page tables.  In such case we must *not* return to the faulting insn -
that would repeat the entire thing without making any progress; what we need
instead is to treat that as failed (user) memory access.
Acked-by: Brian Cain <bcain@quicinc.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/hexagon/mm/vm_fault.c

@@ -93,8 +93,11 @@ void do_page_fault(unsigned long address, long cause, struct pt_regs *regs)
 
 	fault = handle_mm_fault(vma, address, flags, regs);
 
-	if (fault_signal_pending(fault, regs))
+	if (fault_signal_pending(fault, regs)) {
+		if (!user_mode(regs))
+			goto no_context;
 		return;
+	}
 
 	/* The fault is fully completed (including releasing mmap lock) */
 	if (fault & VM_FAULT_COMPLETED)