thp: prepare change_huge_pmd() for file thp

change_huge_pmd() has assert which is not relvant for file page.  For
shared mapping it's perfectly fine to have page table entry writable,
without explicit mkwrite.

Link: http://lkml.kernel.org/r/1466021202-61880-18-git-send-email-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>