mtd: uclinux: add a comment about why uclinux_ram_map must not be static

I was (at least) the second person trying to fix a warning by sparse, so
document in the code why this is a bad idea and add an extern declaration to
make sparse happy.

Signed-off-by: Uwe Kleine-König <[e-mail]>
Acked-by: Mike Frysinger <[e-mail]>
Acked-by: Greg Ungerer <[e-mail]>
Signed-off-by: Artem Bityutskiy <[e-mail]>

drivers/mtd/maps/uclinux.c

···
 #define MAP_NAME "ram"
 #endif
 
+/*
+ * Blackfin uses uclinux_ram_map during startup, so it must not be static.
+ * Provide a dummy declaration to make sparse happy.
+ */
+extern struct map_info uclinux_ram_map;
+
 struct map_info uclinux_ram_map = {
 	.name = MAP_NAME,
 	.size = 0,