mm, procfs: breakdown RSS for anon, shmem and file in /proc/pid/status

There are several shortcomings with the accounting of shared memory
(SysV shm, shared anonymous mapping, mapping of a tmpfs file).  The
values in /proc/<pid>/status and <...>/statm don't allow to distinguish
between shmem memory and a shared mapping to a regular file, even though
theirs implication on memory usage are quite different: during reclaim,
file mapping can be dropped or written back on disk, while shmem needs a
place in swap.

Also, to distinguish the memory occupied by anonymous and file mappings,
one has to read the /proc/pid/statm file, which has a field for the file
mappings (again, including shmem) and total memory occupied by these
mappings (i.e.  equivalent to VmRSS in the <...>/status file.  Getting
the value for anonymous mappings only is thus not exactly user-friendly
(the statm file is intended to be rather efficiently machine-readable).

To address both of these shortcomings, this patch adds a breakdown of
VmRSS in /proc/<pid>/status via new fields RssAnon, RssFile and
RssShmem, making use of the previous preparatory patch.  These fields
tell the user the memory occupied by private anonymous pages, mapped
regular files and shmem, respectively.  Other existing fields in /status
and /statm files are left without change.  The /statm file can be
extended in the future, if there's a need for that.

Example (part of) /proc/pid/status output including the new Rss* fields:

VmPeak:  2001008 kB
VmSize:  2001004 kB
VmLck:         0 kB
VmPin:         0 kB
VmHWM:      5108 kB
VmRSS:      5108 kB
RssAnon:              92 kB
RssFile:            1324 kB
RssShmem:           3692 kB
VmData:      192 kB
VmStk:       136 kB
VmExe:         4 kB
VmLib:      1784 kB
VmPTE:      3928 kB
VmPMD:        20 kB
VmSwap:        0 kB
HugetlbPages:          0 kB

[vbabka@suse.cz: forward-porting, tweak changelog]
Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>