login: ignore SIGXFSZ when write to lastlog

the lastlog file is huge and on systems with large UIDs, it's so huge that
it generates SIGXFSZ when the FSIZE limit is too small.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1165702
Signed-off-by: Karel Zak <kzak@redhat.com>