crimson/osd: use store-specific max_object_size for the OSD-layer write check

crimson/osd: use store-specific max_object_size for the OSD-layer write check

is_offset_and_length_valid() checked write sizes against
osd_max_object_size (128 MiB), but SeaStore caps per-onode laddr space
at seastore_default_max_object_size (16 MiB).  Writes between the two
limits pass the OSD check, reach SeaStore, and trip
prepare_data_reservation()'s ceph_assert(), crashing the OSD and its
replicas.

Add FuturizedStore::Shard::get_max_object_size() (returns
osd_max_object_size by default) and override it in SeaStore::Shard to
return min(osd_max_object_size, max_object_size).  Convert
is_offset_and_length_valid() from a static function to a PGBackend
member that queries the store, so EFBIG reaches the client before the
write ever hits the store.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>