From: Stephen Smalley <sds@epoch.ncsc.mil>

This patch against 2.5.73 replaces vm_enough_memory with a security hook
per Alan Cox's suggestion so that security modules can completely replace
the logic if desired.

Note that the patch changes the interface to follow the convention of the
other security hooks, i.e.  return 0 if ok or -errno on failure (-ENOMEM in
this case) rather than returning a boolean.  It also exports various
variables and functions required for the vm_enough_memory logic.