x86: pku: fix the test to work with new allocator

The test used to simply take a chunk of memory and use it, hoping the
memory allocator would never touch it. The memory area used was exactly
at the beginning of the 16M boundary.

The new allocator stores metadata information there, and could cause the
test to fail.

This patch uses the new features of the allocator to properly reserve
a memory block. To make things easier and cleaner, the memory area used
is now smaller and starts at 8M.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20210121111808.619347-3-imbrenda@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>