coredump: warn about unsafe suid_dumpable / core_pattern combo

coredump: warn about unsafe suid_dumpable / core_pattern combo

When suid_dumpable=2, detect unsafe core_pattern settings and warn when
they are seen.

Signed-off-by: Kees Cook <[email protected]>
Suggested-by: Andrew Morton <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Alan Cox <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
Cc: Doug Ledford <[email protected]>
Cc: Serge Hallyn <[email protected]>
Cc: James Morris <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>