netconsole: Rename userdata to extradata

Rename "userdata" to "extradata" since this structure will hold both
user and system data in future patches. Keep "userdata" term only for
data that comes from userspace (configfs), while "extradata" encompasses
both userdata and future kerneldata.

These are the rules of the design

1. extradata_complete will hold userdata and sysdata (coming)
2. sysdata will come after userdata_length
3. extradata_complete[userdata_length] string will be replaced at every
   message
5. userdata is replaced when configfs changes (update_userdata())
6. sysdata is replaced at every message

Example:
  extradata_complete = "userkey=uservalue cpu=42"
  userdata_length = 17
  sysdata_length = 7 (space (" ") is part of sysdata)

Since sysdata is still not available, you will see the following in the
send functions:

	extradata_len = nt->userdata_length;

The upcoming patches will, which will add support for sysdata, will
change it to:

	extradata_len = nt->userdata_length + sysdata_len;

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>