Problem:
--------
When initializing rollback segment headers(max TRX_SYS_N_RSEGS), all
rollback segment headers are accessed using single mtr.

With buf_pool_size of 5M and innodb_page_size=64k,
the maximum number of pages available is 80.
When a single mtr accesses more than 80 pages, all the blocks
are buffer fixed and cannot be replaced from buf pool.

So we cannot find a free block.

Fix:
----
Access each rollback segment header in a separate mini-transaction.

Reviewed-By: Marko Mäkelä <marko.makela@oracle.com>
RB: 11367