Since mappings established using __MAP_NOFAIL will be converted into anonymous

Since mappings established using __MAP_NOFAIL will be converted into anonymous
memory if the file backing the mapping is truncated, we should check resource
limits.  This prevents callers from triggering a kernel panic and a potential
integer overflow in the amap code by forcing the allocation of too many slots.

Based on an analysis from Jesse Hertz and Tim Newsham.

ok deraadt@