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.com


Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>