fs: optimise kiocb_set_rw_flags()

Use a local var to collect flags in kiocb_set_rw_flags(). That spares
some memory writes and allows to replace most of the jumps with MOVEcc.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>