crimson/seastore: reject oversized writes and zeros instead of aborting

crimson/seastore: reject oversized writes and zeros instead of aborting

prepare_data_reservation() ceph_assert()s the request fits within
seastore_default_max_object_size (16 MiB), but the OSD validates writes
against osd_max_object_size (128 MiB).  Anything between the two limits
passes OSD validation then trips the assert, crashing the OSD and its
replicas.

_zero() already returned EIO for this case; mirror that in _write() and
fix _zero()'s off-by-one (>= should be >, matching the <= in the assert).

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