librbd: check return code and error out if invalidate_cache fails

librbd: check return code and error out if invalidate_cache fails

This will only happen when shrinking or rolling back an image is done
while other I/O is in flight to the same ImageCtx. This is unsafe, so
return an error before performing the resize or rollback.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
(cherry picked from commit e08b8b66c77be3a3d7f79d91c20b1619571149ee)