Since MySQL 5.6.16 (and MariaDB Server 10.0.11), changes of
buf_page_t::buf_fix_count are atomic memory operations if
PAGE_ATOMIC_REF_COUNT is defined. Since MySQL 5.7
(and MariaDB Server 10.2.2), the field is always updated
by atomic memory operations.

In a few occurrences, updates of the counter were unnecessarily
surrounded by an acquisition and release of the block mutex
(buf_block_t::mutex or buf_pool_t::zip_mutex). Remove these
unnecessary mutex operations.