MFC: r291035
The problem report was for a crash that happened when smbfs was
trying to do a mount. Given the backtrace,
it appears that the crash occurred when smb_vc_create() failed and then
called smb_vc_put() with vcp->vc_iod == NULL. smb_vc_put() subsequently
called smb_vc_disconnect() with vcp->vc_iod == NULL, causing the crash.
This patch adds a check for vcp->vc_iod != NULL in smb_vc_disconnect() to
avoid the crash. It also fixes the case in smb_vc_create() where
kproc_create() fails so that it destroys the mutexes and sets
vcp->vc_iod == NULL before free()'ing the iod structure.