mm: unexport follow_pte_pmd

Patch series "simplify follow_pte a bit".

This small series drops the not needed follow_pte_pmd exports, and
simplifies the follow_pte family of functions a bit.

This patch (of 2):

follow_pte_pmd() is only used by the DAX code, which can't be modular.

Link: https://lkml.kernel.org/r/20201029101432.47011-2-hch@lst.deSigned-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -4798,7 +4798,6 @@ int follow_pte_pmd(struct mm_struct *mm, unsigned long address,
 						    ptepp, pmdpp, ptlp)));
 	return res;
 }
-EXPORT_SYMBOL(follow_pte_pmd);
 
 /**
  * follow_pfn - look up PFN at a user virtual address