get_mm_hiwater_xxx: trivial, s/define/inline/

Andrew pointed out get_mm_hiwater_xxx() evaluate "mm" argument thrice/twice,
make them inline.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Hugh Dickins <hugh@veritas.com>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>