posix-clock: Store file pointer in struct posix_clock_context

File descriptor based pc_clock_*() operations of dynamic posix clocks
have access to the file pointer and implement permission checks in the
generic code before invoking the relevant dynamic clock callback.

Character device operations (open, read, poll, ioctl) do not implement a
generic permission control and the dynamic clock callbacks have no
access to the file pointer to implement them.

Extend struct posix_clock_context with a struct file pointer and
initialize it in posix_clock_open(), so that all dynamic clock callbacks
can access it.

Acked-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Wojtek Wasko <wwasko@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>