osd: revert 'osd max xattr size' limit

Set it to 0 (unlimited) for now.

Backport: dumpling

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit abb88d70643c3a76435b7a9d5b04ff29f7502361)