libsmartcols: reject pathological regex patterns in filter

libsmartcols: reject pathological regex patterns in filter

Consecutive ERE quantifiers (e.g. a++, a**) and nested group
repetitions (e.g. (a+)+) cause glibc regcomp() to allocate gigabytes
for the NFA, triggering OOM on small inputs.

Reject such patterns in filter_compile_param() before they reach
regcomp(), and limit regex pattern size to SCOLS_FILTER_MAX_REGSZ (256
bytes). Document all filter expression limits in scols-filter(5).

Addresses: https://github.com/util-linux/util-linux/pull/4436
Signed-off-by: Karel Zak <kzak@redhat.com>