osd: fix detection of non-existent class method

Signed-off-by: Sage Weil <sage@inktank.com>
osd: EINVAL when rmw_flags is 0

A broken client (e.g., v0.56) can send a request that ends up with an
rmw_flags of 0.  Treat this as invalid and return EINVAL.

Fixes: #4556
Signed-off-by: Sage Weil <sage@inktank.com>