mm: do not export ioremap_page_range symbol for external module

Recently, I've found cases in which ioremap_page_range was used
incorrectly, in external modules, leading to crashes.  This can be
partly attributed to the fact that ioremap_page_range is lower-level,
with fewer protections, as compared to the other functions that an
external module would typically call.  Those include:

     ioremap_cache
     ioremap_nocache
     ioremap_prot
     ioremap_uc
     ioremap_wc
     ioremap_wt

...each of which wraps __ioremap_caller, which in turn provides a safer
way to achieve the mapping.

Therefore, stop EXPORT-ing ioremap_page_range.

Link: http://lkml.kernel.org/r/1485173220-29010-1-git-send-email-zhongjiang@huawei.comSigned-off-by: Nzhong jiang <zhongjiang@huawei.com>
Reviewed-by: NJohn Hubbard <jhubbard@nvidia.com>
Suggested-by: NJohn Hubbard <jhubbard@nvidia.com>
Acked-by: NMichal Hocko <mhocko@suse.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

lib/ioremap.c

@@ -144,4 +144,3 @@ int ioremap_page_range(unsigned long addr,
 
 	return err;
 }
-EXPORT_SYMBOL_GPL(ioremap_page_range);