btrfs: don't stash uring encoded data across -EAGAIN

btrfs: don't stash uring encoded data across -EAGAIN

Returning -EAGAIN while leaving btrfs_uring_encoded_data in the cmd PDU
leaks if the request is cancelled or the ring exits before reissue.
io_uring does not free driver PDU allocations on cleanup.

Write: io_queue_sqe() always issues with IO_URING_F_NONBLOCK first, so
return -EAGAIN before allocating and free data on every exit.

Read: free on nowait -EAGAIN too; only -EIOCBQUEUED keeps the
allocation for btrfs_uring_read_finished().

Fixes: 34310c442e17 ("btrfs: add io_uring command for encoded reads (ENCODED_READ ioctl)")
Fixes: e32dcdb0af9f ("btrfs: add io_uring interface for encoded writes")
Signed-off-by: Yang Xiuwei <yangxiuwei@kylinos.cn>
Signed-off-by: David Sterba <dsterba@suse.com>