osd: print op prior to calling methods to check for caps

Signed-off-by: Sage Weil <sage@inktank.com>
MOSDOp: rmw_flags may actually be 0

If the operation includes only an invalid method,
the flags will be 0.

Backported from c313423cfda55a2231e000cd5ff20729310867f8.

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>
Signed-off-by: Samuel Just <sam.just@inktank.com>