io_uring: add missing lock in io_get_file_fixed

io_get_file_fixed will access io_uring's context. Lock it if it is
invoked unlocked (eg via io-wq) to avoid a race condition with fixed
files getting unregistered.

No single upstream patch exists for this issue, it was fixed as part
of the file assignment changes that went into the 5.18 cycle.

Signed-off-by: Jheng, Bing-Jhong Billy <billy@starlabs.sg>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>