proc: copy_to_user() returns unsigned

copy_to_user() returns the number of bytes left to be copied.

This was a typo from: d82ef020 "proc: pagemap: Hold mmap_sem during
page walk".

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>