9p: remove Ron Minnich from MAINTAINERS

Ron Minnich has left Sandia in 2011, and has not been involved in any 9p
commit in recent years.  Also add a CREDITS entry to record his
contributions.

Link: http://lkml.kernel.org/r/1534486244-1055-1-git-send-email-asmadeus@codewreck.org


Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Ron Minnich <rminnich@sandia.gov>
Cc: Ronald G. Minnich <rminnich@gmail.com>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>