MAINTAINERS, SLAB: add Roman as reviewer, git tree

The slab code has an overlap with kmem accounting, where Roman has done
a lot of work recently and it would be useful to make sure he's CC'd on
patches that potentially affect it. Thus add him as a reviewer for the
SLAB subsystem.

Also while at it, add the link to slab git tree.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Vlastimil Babka <[e-mail]>
Acked-by: David Rientjes <[e-mail]>
Acked-by: Roman Gushchin <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Cc: Pekka Enberg <[e-mail]>
Cc: Joonsoo Kim <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

MAINTAINERS

···
 M:	Joonsoo Kim <[e-mail]>
 M:	Andrew Morton <[e-mail]>
 M:	Vlastimil Babka <[e-mail]>
+R:	Roman Gushchin <[e-mail]>
 L:	[e-mail]
 S:	Maintained
+T:	git git://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git
 F:	include/linux/sl?b*.h
 F:	mm/sl?b*