[PATCH] shm: make sysv ipc shared memory use stacked files

The current ipc shared memory code runs into several problems because it
does not quite use files like the rest of the kernel.  With the option of
backing ipc shared memory with either hugetlbfs or ordinary shared memory
the problems got worse.  With the added support for ipc namespaces things
behaved so unexpected that we now have several bad namespace reference
counting bugs when using what appears at first glance to be a reasonable
idiom.

So to attack these problems and hopefully make the code more maintainable
this patch simply uses the files provided by other parts of the kernel and
builds it's own files out of them.  The shm files are allocated in do_shmat
and freed when their reference count drops to zero with their last unmap.
The file and vm operations that we don't want to implement or we don't
implement completely we just delegate to the operations of our backing
file.

This means that we now get an accurate shm_nattch count for we have a
hugetlbfs inode for backing store, and the shm accounting of last attach
and last detach time work as well.

This means that getting a reference to the ipc namespace when we create the
file and dropping the referenece in the release method is now safe and
correct.

This means we no longer need a special case for clearing VM_MAYWRITE
as our file descriptor now only has write permissions when we have
requested write access when calling shmat.  Although VM_SHARED is now
cleared as well which I believe is harmless and is mostly likely a
minor bug fix.

By using the same set of operations for both the hugetlb case and regular
shared memory case shmdt is not simplified and made slightly more correct
as now the test "vma->vm_ops == &shm_vm_ops" is 100% accurate in spotting
all shared memory regions generated from sysvipc shared memory.
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Michal Piotrowski <michal.k.k.piotrowski@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>