add a poll variant to DmaStreamReader::get_buffer_aligned

[HippoBaro]

What does this PR do?

Related to #445 and #446

Since DmaStreamReader is a stream, we have to play well with other
streams and make our functions easy to use from a polling context.
get_buffer_aligned is an async function and, as such, is hard to use
from there (not impossible, just hard).

If we look at the AsyncRead trait, one may see that it defines only
poll functions. Async functions are then built on top in the
AsyncReadExt. This is done because creating a future from a poll
function is trivial, while the other way around is hard.

Therefore, we create a new function poll_get_buffer_aligned and we
reimplement get_buffer_aligned as a wrapper around it:

pub async fn get_buffer_aligned(&mut self, len: u64) -> Result<ReadResult> {
    poll_fn(|cx| self.poll_get_buffer_aligned(cx, len)).await
}

[github-actions]

Greetings @HippoBaro!

It looks like your PR added a new or changed an existing dependency, and CI has failed to validate your changes.
Some possible reasons this could happen:

• One of the dependencies you added uses a restricted license. See deny.toml for a list of licenses we allow;
• One of the dependencies you added has a known security vulnerability;
• You added or updated a dependency and didn't update the LICENSE-3rdparty.csv file. To do so, run the following and commit the changes:

$ cargo install cargo-license
$ cargo license --all-features -a -j --no-deps -d | jq -r '(["Component","Origin","License","Copyright"]) as $cols | map(. as $row | ["name", "repository", "license", "authors"] | map($row[.])) as $rows | $cols, $rows[] | @csv' > LICENSE-3rdparty.csv

Thank you!

[github-actions]

Greetings @HippoBaro!

It looks like your PR added a new or changed an existing dependency, and CI has failed to validate your changes.
Some possible reasons this could happen:

• One of the dependencies you added uses a restricted license. See deny.toml for a list of licenses we allow;
• One of the dependencies you added has a known security vulnerability;
• You added or updated a dependency and didn't update the LICENSE-3rdparty.csv file. To do so, run the following and commit the changes:

$ cargo install cargo-license
$ cargo license --all-features -a -j --no-deps -d | jq -r '(["Component","Origin","License","Copyright"]) as $cols | map(. as $row | ["name", "repository", "license", "authors"] | map($row[.])) as $rows | $cols, $rows[] | @csv' > LICENSE-3rdparty.csv

Thank you!