rgw/datalog: DataLogBackends::trim_entries: fix crash when target_gen > head_gen

rgw/datalog: DataLogBackends::trim_entries: fix crash when target_gen > head_gen

When a cluster has no sync zones (single-zone), DataLogTrimCR passes
max_marker() as the trim marker, which encodes target_gen = UINT64_MAX
from gencursor(). In DataLogBackends::trim_entries, after trimming the
head (last) generation, the break condition

    if (be->gen_id == target_gen)

is false (e.g. 0 != UINT64_MAX), so the loop attempts its increment
expression:

    be = upper_bound(be->gen_id)->second

upper_bound(head_gen) returns end(), and dereferencing end()->second
causes crash.

Fix: also break when be->gen_id >= head_gen. Once we've trimmed the
head generation there are no further backends in the map, so the
upper_bound dereference in the loop increment will be skipped.

This is a general bug that affects any cluster using max_marker() as a
trim target (i.e. every single-zone deployment).

Signed-off-by: Oguzhan Ozmen <oozmen@bloomberg.net>