When this flag is set, operations that update an existing kevent will
not change the udata field.  This can be used to NOTE_TRIGGER or
EV_{EN,DIS}ABLE events without overwriting the stashed pointer.

Reviewed by:	Domagoj Stolfa <domagoj.stolfa@gmail.com>
Obtained from:	CheriBSD
Sponsored by:	Microsoft
Differential Revision:	https://reviews.freebsd.org/D30286