proc: make /proc/pid/limits world readable

Having the limits file world readable will ease the task of system
management on systems where root privileges might be restricted.

Having admin restricted with root priviledges, he/she could not check
other users process' limits.

Also it'd align with most of the /proc stat files.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Cc: Eugene Teo <eugene@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>