io_uring: optimise kiocb_end_write for !ISREG

file_end_write() is only for regular files, so the function do a couple
of dereferences to get inode and check for it. However, we already have
REQ_F_ISREG at hand, just use it and inline file_end_write().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>