rbd: only set STRIPINGV2 feature when needed

Only set the STRIPINGV2 feature if the striping parameters are non-default.
Specifically, fix the case where the passed-in size and count are == 0.

Fixes: #4710
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 5926ffa576e9477324ca00eaec731a224195e7db)

Conflicts:

	src/rbd.cc