kmsg: /proc/kmsg - support reading of partial log records

kmsg: /proc/kmsg - support reading of partial log records

Restore support for partial reads of any size on /proc/kmsg, in case the
supplied read buffer is smaller than the record size.

Some people seem to think is is ia good idea to run:
  $ dd if=/proc/kmsg bs=1 of=...
as a klog bridge.

Resolves-bug: https://bugzilla.kernel.org/show_bug.cgi?id=44211
Reported-by: Jukka Ollila <[email protected]>
Signed-off-by: Kay Sievers <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>