selftests: mm: fix map_hugetlb failure on 64K page size systems

On systems with 64k page size and 512M huge page sizes, the allocation and
test succeeds but errors out at the munmap.  As the comment states, munmap
will failure if its not HUGEPAGE aligned.  This is due to the length of
the mapping being 1/2 the size of the hugepage causing the munmap to not
be hugepage aligned.  Fix this by making the mapping length the full
hugepage if the hugepage is larger than the length of the mapping.

Link: https://lkml.kernel.org/r/20240119131429.172448-1-npache@redhat.com
Signed-off-by: Nico Pache <npache@redhat.com>
Cc: Donet Tom <donettom@linux.vnet.ibm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>