fs/timerfd: Modify last arg of timerfd_ioctl to be user_uintptr_t

Block device ioctl's now take a user_uintptr_t as last argument,
so adapt to this to keep compiling if CONFIG_CHECKPOINT_RESTORE
is enabled.

Signed-off-by: Carsten Haitzler <carsten.haitzler@foss.arm.com>