Fix locking problem in _hash_squeezebucket() / _hash_freeovflpage().

A bucket squeeze operation needs to lock each page of the bucket
before releasing the prior page, but the previous coding fumbled the
locking when freeing an overflow page during a bucket squeeze
operation.  Commit 6d46f478
introduced this bug.

Amit Kapila, with help from Kuntal Ghosh and Dilip Kumar, after
an initial trouble report by Jeff Janes.  Reviewed by me.  I also
fixed a problem with a comment.