arm64/ptdump:display the Linear Mapping start marker

The current /sys/kernel/debug/kernel_page_tables does not display the
*Linear Mapping start* marker on arm64, which I think should be paired
with the *Linear Mapping end* marker.

Since *Linear Mapping start* is the first marker, use initialise 'level'
to -1 in order to display it.

Signed-off-by: Hailong Liu <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Will Deacon <[e-mail]>

arch/arm64/mm/ptdump.c

···
 	st = (struct pg_state){
 		.seq = s,
 		.marker = info->markers,
+		.level = -1,
 		.ptdump = {
 			.note_page = note_page,
 			.range = (struct ptdump_range[]){