sanitize ->mntinfo_list initialization in collect_mntinfo()

Currently this doesn't matter correctness-wise (with or without the
previous changes), but imho collect_mntinfo() needs a cleanup. We
should not return with ->mntinfo_list pointing to the freed memory
on failure, even if currently this failure is fatal and nobody will
ever use this pointer.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Tested-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>