pagemap: fix bug in add_to_pagemap, require aligned-length reads of /proc/pid/pagemap

Fix a bug in add_to_pagemap.  Previously, since pm->out was a char *,
put_user was only copying 1 byte of every PFN, resulting in the top 7
bytes of each PFN not being copied.  By requiring that reads be a multiple
of 8 bytes, I can make pm->out and pm->end u64*s instead of char*s, which
makes put_user work properly, and also simplifies the logic in
add_to_pagemap a bit.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Thomas Tuttle <ttuttle@google.com>
Cc: Matt Mackall <mpm@selenic.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>