crimson/os/seastore/omap_manager: count key length in the overwrite gap

crimson/os/seastore/omap_manager: count key length in the overwrite gap

ow_gap_from_last_entry() returns how much the node grows when the tail entry
is overwritten. However, it compared *value* lengths only.

overwriting pg-log key (31B) is longer than a _fastinfo key (9B) -
So the growth was under-counted by 22 bytes.

Also assert in _append()/_overwrite() that the write stays within capacity() to
avoid similar issues.

Signed-off-by: Matan Breizman <mbreizma@redhat.com>