linux: add definitions for hugetlb page size encodings

A desired hugetlb page size can be encoded in the flags parameter of
system calls such as mmap() and shmget(). The Linux UAPI headers have
included explicit definitions for these encodings since v4.14.

This patch adds these definitions that are used along with MAP_HUGETLB
and SHM_HUGETLB flags as specified in the corresponding man pages. This
relieves programs from having to duplicate and/or compute the encodings
manually.

Additionally, the filter on these definitions in tst-mman-consts.py is
removed, as suggested by Florian. I then ran this tests successfully,
confirming the alignment with the kernel headers.

  PASS: misc/tst-mman-consts
  original exit status 0

Signed-off-by: Carlos Llamas <cmllamas@google.com>
Tested-by: Florian Weimer <fweimer@redhat.com>
Reviewed-by: Florian Weimer <fweimer@redhat.com>