efi: Don't attempt to map RCI2 config table if it doesn't exist

Commit:

  1c5fecb ("efi: Export Runtime Configuration Interface table to sysfs")

... added support for a Dell specific UEFI configuration table, but
failed to take into account that mapping the table should not be
attempted unless the table actually exists. If it doesn't exist,
the code usually fails silently unless pr_debug() prints are
enabled. However, on 32-bit PAE x86, the splat below is produced due
to the attempt to map the placeholder value EFI_INVALID_TABLE_ADDR
which we use for non-existing UEFI configuration tables, and which
equals ULONG_MAX.

   memremap attempted on mixed range 0x00000000ffffffff size: 0x1e
   WARNING: CPU: 1 PID: 1 at kernel/iomem.c:81 memremap+0x1a3/0x1c0
   Modules linked in:
   CPU: 1 PID: 1 Comm: swapper/0 Not tainted 5.4.2-smp-mine #1
   Hardware name: Hewlett-Packard HP Z400 Workstation/0B4Ch, BIOS 786G3 v03.61 03/05/2018
   EIP: memremap+0x1a3/0x1c0
  ...
   Call Trace:
    ? map_properties+0x473/0x473
    ? efi_rci2_sysfs_init+0x2c/0x154
    ? map_properties+0x473/0x473
    ? do_one_initcall+0x49/0x1d4
    ? parse_args+0x1e8/0x2a0
    ? do_early_param+0x7a/0x7a
    ? kernel_init_freeable+0x139/0x1c2
    ? rest_init+0x8e/0x8e
    ? kernel_init+0xd/0xf2
    ? ret_from_fork+0x2e/0x38

Fix this by checking whether the table exists before attempting to map it.

Reported-by: Richard Narron <comet.berkeley@gmail.com>
Tested-by: Richard Narron <comet.berkeley@gmail.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Cc: linux-efi@vger.kernel.org
Fixes: 1c5fecb ("efi: Export Runtime Configuration Interface table to sysfs")
Link: https://lkml.kernel.org/r/20191210090945.11501-2-ardb@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

drivers/firmware/efi/rci2-table.c

@@ -81,6 +81,9 @@ static int __init efi_rci2_sysfs_init(void)
 	struct kobject *tables_kobj;
 	int ret = -ENOMEM;
 
+	if (rci2_table_phys == EFI_INVALID_TABLE_ADDR)
+		return 0;
+
 	rci2_base = memremap(rci2_table_phys,
 			     sizeof(struct rci2_table_global_hdr),
 			     MEMREMAP_WB);