mm: thp: simplify the implementation of mk_huge_pmd()

The implementation of mk_huge_pmd looks verbose, it could be just
simplified to one line code.

Signed-off-by: Yang Shi <yang.shi@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>