In MDEV-15053 (commit b1ab211d)
we inadvertently removed a check whether innodb_flush_neighbors is 0,
and thus started treating only the value 1 in a special way.

buf_flush_check_neighbors(): Add the parameter contiguous,
which can be set to skip the check for non-contiguous page number ranges.

Reviewed by: Thirunarayanan Balathandayuthapani