arm64: Don't walk page table for user faults in do_mem_abort

Commit 42dbf54e ("arm64: consistently log ESR and page table")
dumps page table entries for user faults hitting do_bad entries in the
fault handler table. Whilst this shouldn't really happen in practice,
it's not beyond the realms of possibility if e.g. running an old kernel
on a new CPU.

Generally, we want to avoid exposing physical addresses under the control
of userspace (see commit bf396c09 ("arm64: mm: don't print out page
table entries on EL0 faults")), so walk the page tables only on exceptions
from EL1.
Reported-by: Kristina Martsenko <kristina.martsenko@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/mm/fault.c

@@ -698,6 +698,7 @@ asmlinkage void __exception do_mem_abort(unsigned long addr, unsigned int esr,
 
 	mem_abort_decode(esr);
 
+	if (!user_mode(regs))
 		show_pte(addr);
 
 	info.si_signo = inf->sig;