os: rename get_*() -> decode_*()

These methods have side-effects: they move the decode iterator *and*
return a value.  Rename them to avoid confusion with typical get_*
accessors.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1922 from ceph/wip-os-rename

os: rename get_*() -> decode_*()

Reviewed-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@cloudwatt.com>