mm: make get_mm_counter static-inline

Make get_mm_counter() always static inline, it is simple enough for that.
And remove unused set_mm_counter()

bloat-o-meter:

add/remove: 0/1 grow/shrink: 4/12 up/down: 99/-341 (-242)
function                                     old     new   delta
try_to_unmap_one                             886     952     +66
sys_remap_file_pages                        1214    1230     +16
dup_mm                                      1684    1700     +16
do_exit                                     2277    2278      +1
zap_page_range                               208     205      -3
unmap_region                                 304     296      -8
static.oom_kill_process                      554     546      -8
try_to_unmap_file                           1716    1700     -16
getrusage                                    925     909     -16
flush_old_exec                              1704    1688     -16
static.dump_header                           416     390     -26
acct_update_integrals                        218     187     -31
do_task_stat                                2986    2954     -32
get_mm_counter                                34       -     -34
xacct_add_tsk                                371     334     -37
task_statm                                   172     118     -54
task_mem                                     383     323     -60

try_to_unmap_one() grows because update_hiwater_rss() now completely inline.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>