VM/Security: add security hook to do_brk

Given a specifically crafted binary do_brk() can be used to get low pages
available in userspace virtual memory and can thus be used to circumvent
the mmap_min_addr low memory protection.  Add security checks in do_brk().

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Alan Cox <alan@redhat.com>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: James Morris <jmorris@namei.org>
Cc: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>