pass "bool for_dump" argument down to collect_mntinfo() and parse_mountinfo()

Preparation.

1. Add the new "bool for_dump" arg to collect/parse_mntinfo().

2. Introduce "struct collect_mntns_arg" to pass the additional
   "bool for_dump" field to collect_mntinfo() and change it to
   pass this boolean to collect_mntinfo()->parse_mountinfo() path.

3. Change other callers of collect_mntinfo() to pass "false".

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>