metag: hugetlbpage: Remove null pointer checks that could never happen

There are inconsistent NULL checks in
hugetlb_get_unmapped_area_existing(). The last conditional block in the
loop checks vma, but vma is dereferenced after that.

Due to the checks above however (bearing in mind after_huge is either 0
or 1) vma is guaranteed not to be NULL here, so remove that part of the
check.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
[james.hogan@imgtec.com: remove brackets and expand commit message]
Signed-off-by: James Hogan <james.hogan@imgtec.com>