regression in Bug #54914, but it does speed up the execution for
innodb_change_buffering=inserts.

ibuf_add_ops(), ibuf_merge_or_delete_for_page(),
ibuf_delete_for_discarded_space(): Use atomic built-ins instead of
ibuf_mutex, when available.

ibuf_add_free_page(), ibuf_remove_free_page(), ibuf_contract_ext():
Release ibuf_mutex earlier.

ibuf_free_excess_pages(): Release ibuf_mutex before a conditional branch.

ibuf_insert_low(): Release ibuf_mutex before a conditional
branch. Create ibuf_entry before re-acquiring ibuf_mutex. Simplify a
loop to reduce code footprint. Release ibuf_mutex before mtr_commit()
[btr_pcur_close()].

ibuf_is_empty(): Release ibuf_mutex before mtr_commit().