Clarify mmgrab by collapsing it into get_task_mm (in fork.c not inline),
and commenting on the special case it is guarding against: when use_mm in
an AIO daemon temporarily adopts the mm while it's on its way out.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>