The source address field in the transport's sock_xprt is initialized
ONLY IF the RPC application passed a pointer to a source address
during the call to rpc_create().  However, xs_bind() subsequently uses
the value of this field without regard to whether the source address
was initialized during transport creation or not.

So far we've been lucky: the uninitialized value of this field is
zeroes.  xs_bind(), until recently, used only the sin[6]_addr field in
this sockaddr, and all zeroes is a valid value for this: it means
ANYADDR.  This is a happy coincidence.

However, xs_bind() now wants to use the sa_family field as well, and
expects it to be initialized to something other than zero.

Therefore, the source address sockaddr field should be fully
initialized at transport create time in _every_ case, not just when
the RPC application wants to use a specific bind address.

Bruce added a workaround for this missing initialization by adjusting
commit 6bc9638a, but the "right" way to do this is to ensure that the
source address sockaddr is always correctly initialized from the
get-go.

This patch doesn't introduce a behavior change.  It's simply a
clean-up of Bruce's fix, to prevent future problems of this kind.  It
may look like overkill, but

  a) it clearly documents the default initial value of this field,

  b) it doesn't assume that the sockaddr_storage memory is first
     initialized to any particular value, and

  c) it will fail verbosely if some unknown address family is passed
     in

Originally introduced by commit d3bc9a1d.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>