[PATCH] CPT: check ctx->file for NULL
We need to be sure that dumpfile pointer (ctx->file) is not NULL, because we
can't start dump without it.
Also we need to return error like EINTR instead of ERESTART*, because we just
can't simply restart dump ioctl. The reason is that dumpfile is already
closed and we need to reopen it before calling dump ioctl second time.
These bugs can be easily triggered on RH5 if we will run
strace -f vzctl chkpnt VEID --dump