libmount: fix mtab update for "none" source, fix leak

tab_parse.c:mnt_parse_mountinfo_line parses "none" in src as NULL,

tab_update.c:fprintf_mtab_fs sets m1 to NULL instead of "none" and
returns -ENOMEM
tab_update.c:update_table says "write entry failed: Success", as errno
hasn't been set, and gotos to leave, leaving mtab not updated.

Addresses-debian-bug: 634871
Reported-by: Tomas Janousek <tomi@nomi.cz>
Signed-off-by: Karel Zak <kzak@redhat.com>