In xbd_connect(), use correct scanf conversion specifiers for the
feature_barrier and feature_flush variables. Otherwise, adjacent
variables on the stack, such as sector_size, may be overwritten, with
disastrous results.

Note that I did not see a good reason to revert the addition of zero
checks introduced in r310013. Better safe than sorry.

PR: 215209
Tested by: royger
MFC after: 3 days