UPSTREAM: tracing/boot: Fix to loop on only subkeys

Since the commit e5efaeb8 ("bootconfig: Support mixing
a value and subkeys under a key") allows to co-exist a value
node and key nodes under a node, xbc_node_for_each_child()
is not only returning key node but also a value node.
In the boot-time tracing using xbc_node_for_each_child() to
iterate the events, groups and instances, but those must be
key nodes. Thus it must use xbc_node_for_each_subkey().

Link: https://lkml.kernel.org/r/163112988361.74896.2267026262061819145.stgit@devnote2



Fixes: e5efaeb8 ("bootconfig: Support mixing a value and subkeys under a key")
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
(cherry picked from commit cfd79983)
Bug: 187129171
Signed-off-by: Connor O'Brien <connoro@google.com>
Change-Id: I7cf6b7fbf334edc44e6da1f519f30d467ece6fec