coredump: clarify "unsafe core_pattern" warning

coredump: clarify "unsafe core_pattern" warning

I was amused to find "unsafe core_pattern" warning having these lines in
/etc/sysctl.conf:

	fs.suid_dumpable=2
	kernel.core_pattern=/core/core-%e-%p-%E
	kernel.core_uses_pid=0

Turns out kernel is formally right.  Default core_pattern is just "core",
which doesn't qualify for secure path while setting suid.dumpable.

Hint admins about solution, clarify sysctl names, delete unnecessary '\'
characters (string literals are concatenated regardless) and reformat for
easier grepping.

Link: http://lkml.kernel.org/r/20161029152124.GA1258@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>