net: netconsole: add a userdata config_group member to netconsole_target

Create configfs machinery for netconsole userdata appending, which depends
on CONFIG_NETCONSOLE_DYNAMIC (for configfs interface). Add a userdata
config_group to netconsole_target for managing userdata entries as a tree
under the netconsole configfs subsystem. Directory names created under the
userdata directory become userdatum keys; the userdatum value is the
content of the value file.

Include the minimum-viable-changes for userdata configfs config_group.
init_target_config_group() ties in the complete configfs machinery to
avoid unused func/variable errors during build. Initializing the
netconsole_target->group is moved to init_target_config_group, which
will also init and add the userdata config_group.

Each userdatum entry has a limit of 256 bytes (54 for
the key/directory, 200 for the value, and 2 for '=' and '\n'
characters), which is enforced by the configfs functions for updating
the userdata config_group.

When a new netconsole_target is created, initialize the userdata
config_group and add it as a default group for netconsole_target
config_group, allowing the userdata configfs sub-tree to be presented
in the netconsole configfs tree under the userdata directory.

Co-developed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Matthew Wood <thepacketgeek@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>