fork: fix pidfd_poll()'s return type

fork: fix pidfd_poll()'s return type

pidfd_poll() is defined as returning 'unsigned int' but the
.poll method is declared as returning '__poll_t', a bitwise type.

Fix this by using the proper return type and using the EPOLL
constants instead of the POLL ones, as required for __poll_t.

Fixes: b53b0b9d9a61 ("pidfd: add polling support")
Cc: Joel Fernandes (Google) <[email protected]>
Cc: [email protected] # 5.3
Signed-off-by: Luc Van Oostenryck <[email protected]>
Reviewed-by: Christian Brauner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Brauner <[email protected]>