MAINTAINERS: add Lorenzo as vmalloc reviewer

I have recently been involved in both reviewing and submitting patches to
the vmalloc code in mm and would be willing and happy to help out with
review going forward if it would be helpful!

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Lorenzo Stoakes <[e-mail]>
Acked-by: Uladzislau Rezki (Sony) <[e-mail]>
Cc: Baoquan He <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

MAINTAINERS

···
 M:	Andrew Morton <[e-mail]>
 R:	Uladzislau Rezki <[e-mail]>
 R:	Christoph Hellwig <[e-mail]>
+R:	Lorenzo Stoakes <[e-mail]>
 L:	[e-mail]
 S:	Maintained
 W:	http://www.linux-mm.org