btrfs: ioctl: don't free iov when btrfs_encoded_read() returns -EAGAIN

Fix a bug in encoded read that mistakenly frees the iov in case
btrfs_encoded_read() returns -EAGAIN assuming the structure will be
reused.  This can happen when when receiving requests concurrently, the
io_uring subsystem does not reset the data, and the last free will
happen in btrfs_uring_read_finished().

Handle the -EAGAIN error and skip freeing iov.

CC: stable@vger.kernel.org # 6.13+
Signed-off-by: Sidong Yang <sidong.yang@furiosa.ai>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>