MAINTAINERS: include linux-mm for xarray maintenance

MM developers have an interest in the xarray code.

Cc: David Gow <[e-mail]>
Cc: Geert Uytterhoeven <[e-mail]>
Cc: "Liam R. Howlett" <[e-mail]>
Cc: Lorenzo Stoakes <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Sidhartha Kumar <[e-mail]>
Cc: Tamir Duberstein <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

MAINTAINERS

···
 XARRAY
 M:	Matthew Wilcox <[e-mail]>
 L:	[e-mail]
+L:	[e-mail]
 S:	Supported
 F:	Documentation/core-api/xarray.rst
 F:	include/linux/idr.h