pagemap: do not leak physical addresses to non-privileged userspace

pagemap: do not leak physical addresses to non-privileged userspace

As pointed by recent post[1] on exploiting DRAM physical imperfection,
/proc/PID/pagemap exposes sensitive information which can be used to do
attacks.

This disallows anybody without CAP_SYS_ADMIN to read the pagemap.

[1] http://googleprojectzero.blogspot.com/2015/03/exploiting-dram-rowhammer-bug-to-gain.html

[ Eventually we might want to do anything more finegrained, but for now
  this is the simple model.   - Linus ]

Signed-off-by: Kirill A. Shutemov <[email protected]>
Acked-by: Konstantin Khlebnikov <[email protected]>
Acked-by: Andy Lutomirski <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Mark Seaborn <[email protected]>
Cc: [email protected]
Signed-off-by: Linus Torvalds <[email protected]>