regmap: fix kernel hang on regmap_bulk_write with zero val_count.

Fixes commit 2f06fa04 which was an
incorrect backported version of commit
d6b41cb0 upstream.

If val_count is zero we return -EINVAL with map->lock_arg locked, which
will deadlock the kernel next time we try to acquire this lock.

This was introduced by f5942dd ("regmap: fix possible ZERO_SIZE_PTR pointer
dereferencing error.") which improperly back-ported d6b41cb0.

This issue was found during review of Ubuntu Trusty 3.13.0-40.68 kernel to
prepare Ksplice rebootless updates.

Fixes: f5942dd ("regmap: fix possible ZERO_SIZE_PTR pointer dereferencing error.")
Signed-off-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/regmap/regmap.c

@@ -1557,8 +1557,10 @@ int regmap_bulk_write(struct regmap *map, unsigned int reg, const void *val,
 	} else {
 		void *wval;
 
-		if (!val_count)
-			return -EINVAL;
+		if (!val_count) {
+			ret = -EINVAL;
+			goto out;
+		}
 
 		wval = kmemdup(val, val_count * val_bytes, GFP_KERNEL);
 		if (!wval) {