Btrfs: add a helper to retrive extent inline ref type

Btrfs: add a helper to retrive extent inline ref type

An invalid value of extent inline ref type may be read from a
malicious image which may force btrfs to crash.

This adds a helper which does sanity check for the ref type, so we can
know if it's sane, return he type, otherwise return an error.

Signed-off-by: Liu Bo <[email protected]>
Reviewed-by: David Sterba <[email protected]>
[ minimal tweak const types, causing warnings due to other cleanup patches ]
Signed-off-by: David Sterba <[email protected]>