MAINTAINERS: add an explicit credentials entry

MAINTAINERS: add an explicit credentials entry

The lack of an explicit credential (kernel/cred.c) entry has caused
confusion in the past among new, and not-so-new developers, about where
to send credential patches for discussion and merging.  Those patches
that are sent can often rot on the mailing lists for months as there
is no clear maintainer tasked with reviewing and merging patches.

I'm volunteering for the cred maintainer role to try and reduce the
confusion and help cred patches find their way up to Linus' tree.  As
there generally aren't a lot of cred patches I'll start with simply
folding them into the LSM tree, but if this changes I'll setup a
dedicated cred tree.

Link: https://lore.kernel.org/all/20250304222304.214704-2-paul@paul-moore.com
Signed-off-by: Paul Moore <paul@paul-moore.com>