Per user rlimits

[Eric W. Biederman]

Just to scope how much work it would be to fix rlimits
so they are not a problem for user namespaces I took a quick
survey.

The rlimits can be found in
include/uapi/asm-generic/resource.h

There are a total of 16 rlimits.
There are only 4 rlimits that are enforced at anything other
than process granularity.

RLIMIT_NPROC
RLIMIT_MEMLOCK 
RLIMIT_SIGPENDING
RLIMIT_MSGQUEUE

So it should not be difficult to fix those rlimits.

I think the implementation of RLIMIT_MEMLOCK is highly suspect, and
might be worth reexamining, as RLMIT_MEMLOCK it interpreted differently
in different contexts.  For the limit there is mm->locked_vm,
user->lock_vm, and user->locked_shm.

Eric