ktime: Add us_to_ktime()

Add a us_to_ktime() helper to go with ms_to_ktime() and ns_to_ktime().

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Thomas Gleixner <tglx@linutronix.de>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20241204074710.990092-2-dhowells@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>