arm64/mm: Drop mmap_sem before calling __do_kernel_fault()

There is an inconsistency between down_read_trylock() success and failure
paths while dealing with kernel access for non exception table areas where
it calls __do_kernel_fault(). In case of failure it just bails out without
holding mmap_sem but when it succeeds it does so while holding mmap_sem.
Fix this inconsistency by just dropping mmap_sem in success path as well.

__do_kernel_fault() calls die_kernel_fault() which then calls show_pte().
show_pte() in this path might become bit more unreliable without holding
mmap_sem. But there are already instances [1] in do_page_fault() where
die_kernel_fault() gets called without holding mmap_sem. show_pte() can
be made more robust independently but in a later patch.

[1] Conditional block for (is_ttbr0_addr && is_el1_permission_fault)

Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/mm/fault.c

@@ -504,8 +504,10 @@ retry:
 		 */
 		might_sleep();
 #ifdef CONFIG_DEBUG_VM
-		if (!user_mode(regs) && !search_exception_tables(regs->pc))
+		if (!user_mode(regs) && !search_exception_tables(regs->pc)) {
+			up_read(&mm->mmap_sem);
 			goto no_context;
+		}
 #endif
 	}