kernel-doc: ignore kmemcheck_bitfield_begin/end

Teach kernel-doc to ignore kmemcheck_bitfield_{begin,end} sugar
so that it won't generate warnings like this:

Warning(include/net/sock.h:297): No description found for parameter 'kmemcheck_bitfield_begin(flags)'
Warning(include/net/sock.h:297): No description found for parameter 'kmemcheck_bitfield_end(flags)'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>