Fix build with !HUGETLBFS

I stupidly broke the case of CONFIG_HUGETLBFS=n when doing the
conversion to vm_flags_t in commit ca16d140

 ("mm: don't access
vm_flags as 'int'").  And my 'allyesconfig' build didn't find it, for
obvious reasons..

Include <linux/mm_types.h> in <linux/hugetlb.h>.  The problem could have
been avoided by just turning the hugetlb_file_setup() error wrapper into
a macro, but mm_types.h is a reasonable include in this file.

Reported-by: Richard -rw- Weinberger <richard.weinberger@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>