mount: don't dereference pointer before null check

CID 190775 (#1 of 1): Dereference before null check (REVERSE_INULL)
check_after_deref: Null-checking m->parent suggests that it may be null,
but it has already been dereferenced on all paths leading to the check.

Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>