mm: polish virtual memory accounting

* add VM_STACK as alias for VM_GROWSUP/DOWN depending on architecture
* always account VMAs with flag VM_STACK as stack (as it was before)
* cleanup classifying helpers
* update comments and documentation

Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
Tested-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/proc.txt

@@ -240,8 +240,8 @@ Table 1-2: Contents of the status files (as of 4.1)
  RssFile                     size of resident file mappings
  RssShmem                    size of resident shmem memory (includes SysV shm,
                              mapping of tmpfs and shared anonymous mappings)
- VmData                      size of data, stack, and text segments
- VmStk                       size of data, stack, and text segments
+ VmData                      size of private data segments
+ VmStk                       size of stack segments
  VmExe                       size of text segment
  VmLib                       size of shared library code
  VmPTE                       size of page table entries