FIELD->FIXED_LEN == FIELD->PREFIX_LEN

Background:
-----------
For fixed columns maximum length allowed is 255 (.i.e CHAR(255)). With
charset utf8mb4 or utf32, each char can take at max 4 bytes.
So the maximum possible fixed size is (255*4) 1020 bytes.

when key field length is > 767 bytes, it is chosen as long fixed length
field and treated as variable length field and this field can be chosen
for off-page storage.

Problem:
--------
When such long length fixed field is treated as variable length field,
and when estimating the maximum the record size in a btree index page,
we wrongly treat it as fixed length field.

Fix:
----
If long length fixed fields are treated as variable-length fields, do
not treat them as fixed size fields when estimating the the maximum record
size of a btree index page

Reviewd-by: Marko Makela <marko.makela@oracle.com>
RB: 10375