FDPIC: Don't attempt to expand the userspace stack to fill the space allocated

Stop the ELF-FDPIC binfmt from attempting to expand the userspace stack and brk
segments to fill the space actually allocated for it.  The space allocated may
be rounded up by mmap(), and may be wasted.

However, finding out how much space we actually obtained uses the contentious
kobjsize() function which we'd like to get rid of as it doesn't necessarily
work for all slab allocators.

Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Mike Frysinger <vapier.adi@gmail.com>
Acked-by: Paul Mundt <lethal@linux-sh.org>