slab: Update MAINTAINERS entry

slab: Update MAINTAINERS entry

Add the two Documentation files to the maintainers entry.  Simplify the
match for header files now that there is only slab.h.  Move Vlastimil
to the top of the list of maintainers since he's the one doing the pull
requests.  Change David & Christoph's roles to Reviewer.  Expand the
pattern for files in mm/ so that somebody searching for 'slub' will
find it.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Harry Yoo <harry.yoo@oracle.com>
Acked-by: David Rientjes <rientjes@google.com>
Link: https://patch.msgid.link/20250611155916.2579160-12-willy@infradead.org
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>