io_uring: correct __must_hold annotation in io_install_fixed_file

The __must_hold annotation references &req->ctx->uring_lock, but req
is not in scope in io_install_fixed_file. This change updates the
annotation to reference the correct ctx->uring_lock.
improving code clarity.

Fixes: f110ed84 ("io_uring: split out fixed file installation and removal")
Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>