osd: read into correct variable for magic string

4d140a71a1a48081b449b7d8dde808eb6e74c6b2 refactored this and
introduced a bug. peek_meta() was accidentally reading into magic,
then replacing magic with val, which was always the empty string,
resulting in the osd always failing to start due to 'mismatched'
magic values.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge pull request #887 from jdurgin/wip-peek-meta

osd: read into correct variable for magic string

Reviewed-by: Sage Weil <sage@inktank.com>