mm: backing-dev: set variables dev_attr_min,max_bytes storage-class-specifier to static

smatch reports
mm/backing-dev.c:266:1: warning: symbol
  'dev_attr_min_bytes' was not declared. Should it be static?
mm/backing-dev.c:294:1: warning: symbol
  'dev_attr_max_bytes' was not declared. Should it be static?

These variables are only used in one file so should be static.

Link: https://lkml.kernel.org/r/20230408141609.2703647-1-trix@redhat.comSigned-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/backing-dev.c

@@ -263,7 +263,7 @@ static ssize_t min_bytes_store(struct device *dev,
 
 	return ret;
 }
-DEVICE_ATTR_RW(min_bytes);
+static DEVICE_ATTR_RW(min_bytes);
 
 static ssize_t max_bytes_show(struct device *dev,
 			      struct device_attribute *attr,
@@ -291,7 +291,7 @@ static ssize_t max_bytes_store(struct device *dev,
 
 	return ret;
 }
-DEVICE_ATTR_RW(max_bytes);
+static DEVICE_ATTR_RW(max_bytes);
 
 static ssize_t stable_pages_required_show(struct device *dev,
 					  struct device_attribute *attr,