eventpoll: Annotate data-race of busy_poll_usecs

A struct eventpoll's busy_poll_usecs field can be modified via a user
ioctl at any time. All reads of this field should be annotated with
READ_ONCE.

Fixes: 85455c79 ("eventpoll: support busy poll per epoll instance")
Cc: stable@vger.kernel.org
Signed-off-by: Martin Karsten <mkarsten@uwaterloo.ca>
Link: https://lore.kernel.org/r/20240806123301.167557-1-jdamato@fastly.com


Reviewed-by: Joe Damato <jdamato@fastly.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>