mm: use down_read_killable for locking mmap_sem in access_remote_vm

This function is used by ptrace and proc files like /proc/pid/cmdline and
/proc/pid/environ.

Access_remote_vm never returns error codes, all errors are ignored and
only size of successfully read data is returned.  So, if current task was
killed we'll simply return 0 (bytes read).

Mmap_sem could be locked for a long time or forever if something goes
wrong.  Using a killable lock permits cleanup of stuck tasks and
simplifies investigation.

Link: http://lkml.kernel.org/r/156007494202.3335.16782303099589302087.stgit@buzz


Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>