report fd/path if dump_one_reg_file()->lookup_nsid_by_mnt_id() fails

"Unable to look up the %d mount" doesn't really help to understand
the problem, add a bit more info.

And perhaps it makes more sense to change dump_task_files_seized()
to report fd/path if dump_one_file() fails.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

files-reg.c

@@ -818,7 +818,8 @@ int dump_one_reg_file(int lfd, u32 id, const struct fd_parms *p)
 
 	nsid = lookup_nsid_by_mnt_id(p->mnt_id);
 	if (nsid == NULL) {
-		pr_err("Unable to look up the %d mount\n", p->mnt_id);
+		pr_err("Can't lookup mount=%d for fd=%d path=%s\n",
+			p->mnt_id, p->fd, link->name + 1);
 		return -1;
 	}