ACPI: NFIT: Fix ARS zero-sized allocation

Pending commit in -next "devres: handle zero size in devm_kmalloc()"
triggers a boot regression due to the ARS implementation expecting NULL
from a zero-sized allocation. Avoid the zero-sized allocation by
skipping ARS, otherwise crashes with the following signature when
de-referencing ZERO_SIZE_PTR.

     BUG: kernel NULL pointer dereference, address: 0000000000000018
     #PF: supervisor read access in kernel mode
     #PF: error_code(0x0000) - not-present page
     RIP: 0010:__acpi_nfit_scrub+0x28a/0x350 [nfit]
     [..]
     Call Trace:
       ? acpi_nfit_query_poison+0x6a/0x180 [nfit]
       acpi_nfit_scrub+0x36/0xb0 [nfit]
       process_one_work+0x23c/0x580
       worker_thread+0x50/0x3b0

Otherwise the implementation correctly aborts when NULL is returned from
devm_kzalloc() in ars_status_alloc().

Link: https://lore.kernel.org/r/159624590643.3037264.14157533719042907758.stgit@dwillia2-desk3.amr.corp.intel.com
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>

drivers/acpi/nfit/core.c

@@ -3334,7 +3334,7 @@ static void acpi_nfit_init_ars(struct acpi_nfit_desc *acpi_desc,
 static int acpi_nfit_register_regions(struct acpi_nfit_desc *acpi_desc)
 {
 	struct nfit_spa *nfit_spa;
-	int rc;
+	int rc, do_sched_ars = 0;
 
 	set_bit(ARS_VALID, &acpi_desc->scrub_flags);
 	list_for_each_entry(nfit_spa, &acpi_desc->spas, list) {
@@ -3346,7 +3346,7 @@ static int acpi_nfit_register_regions(struct acpi_nfit_desc *acpi_desc)
 		}
 	}
 
-	list_for_each_entry(nfit_spa, &acpi_desc->spas, list)
+	list_for_each_entry(nfit_spa, &acpi_desc->spas, list) {
 		switch (nfit_spa_type(nfit_spa->spa)) {
 		case NFIT_SPA_VOLATILE:
 		case NFIT_SPA_PM:
@@ -3354,6 +3354,13 @@ static int acpi_nfit_register_regions(struct acpi_nfit_desc *acpi_desc)
 			rc = ars_register(acpi_desc, nfit_spa);
 			if (rc)
 				return rc;
+
+			/*
+			 * Kick off background ARS if at least one
+			 * region successfully registered ARS
+			 */
+			if (!test_bit(ARS_FAILED, &nfit_spa->ars_state))
+				do_sched_ars++;
 			break;
 		case NFIT_SPA_BDW:
 			/* nothing to register */
@@ -3372,8 +3379,10 @@ static int acpi_nfit_register_regions(struct acpi_nfit_desc *acpi_desc)
 			/* don't register unknown regions */
 			break;
 		}
+	}
 
-	sched_ars(acpi_desc);
+	if (do_sched_ars)
+		sched_ars(acpi_desc);
 	return 0;
 }