fs/binfmt_elf_fdpic.c: fix brk area overlap with stack on NOMMU

On NOMMU archs, the FDPIC ELF loader sets up the usable brk range to
overlap with all but the last PAGE_SIZE bytes of the stack. This leads
to catastrophic memory reuse/corruption if brk is used. Fix by setting
the brk area to zero size to disable its use.

Signed-off-by: Rich Felker <dalias@libc.org>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>