[FIX] base: attachment check linked record 'write'

When an attachment is linked to a record (res_id and res_model are set)
we check the access rights and access rules of that record.

The access we check on linked record has changed as follow:

- 15905e78 (2013) => we check `write` access right/rule for `create`
- f5ebc509 (2014) => we check `write` access right for all mode
- 66644e85 (2015) => we check write access right for all but create mode

So currently we check on the linked record for each mode:

- create: write access right / write access rule
- read:   read access right  / read access rule
- write:  write access right / write access rule
- unlink: write access right / unlink access rule

The behavior is not expected for `unlink`, we should check if we have
write access through access rules instead of checking unlink access.

Without the change, the added test failed with a `unlink` access rule
AccessError on the linked record.

opw-2154448
closes #41814

closes odoo/odoo#42264

X-original-commit: 2b3fc2e3
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>