kern_utimensat: Update name of last arg in prototype.

The last argument is a mask of AT_* flags, not a namei cnp flag as
'int follow' implies in other kern_* functions.

Obtained from: CheriBSD
Sponsored by: The University of Cambridge, Google Inc.

(cherry picked from commit 3225fd22b2191c7a7a655cb5dacea9148f29c926)