net/sock.h: make some fields private to fix kernel-doc warning(s)

Fix new kernel-doc notation warning in sock.h by annotating skc_dontcopy_*
as private fields.

Warning(include/net/sock.h:163): No description found for parameter 'skc_dontcopy_end[0]'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>