io_uring: don't pass around fixed index for scm

There is an old API nuisance where io_uring's SCM accounting functions
traverse fixed file tables and so requires them to be set in advance,
which leads to some implicit rules of how io_sqe_file_register() should
be used.

__io_sqe_files_scm() now works with only one file at a time, pass a file
directly and get rid of all fixed table dereferencing inside. Clean
io_sqe_file_register() callers.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/fb32031d892e61a7748c70da7999725d5e798671.1649334991.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>