WL#14452: InnoDB: Add configuration variable to switch between fdatasync() and fsync()

Affects: Server-8.0   —   Status: Complete

On platforms where fdatasync() is available we should allow the user to switch to using fdatasync() from fsync(). Which is basically Linux.

Proposed configuration parameter: --innodb-use-fdatasync := bool
FR1 - On platforms that have fdatasync defined, setting --innodb-use-fdatasync 
will make InnoDB use fdatasync instead of the default fsync for os buffer flushes.

Introduce a new MySQL variable indicating whether InnoDB should use fdatasync 
instead of fsync for os flushes.This is a dynamic global variable.

  [1] ./bin/mysqld --innodb-use-fdatasync=1

When using this flag, InnoDB will use the fdatasync system call which will not 
flush the metadata of accessed files on each flush. This should allow for some 
performance gains in certain scenarios.
1) Declare the new MYSQL_SYSVAR_BOOL in ha_innodb.cc - use_fdatasync:

static MYSQL_SYSVAR_BOOL(use_fdatasync, srv_use_fdatasync, PLUGIN_VAR_NOCMDARG,
                         "Use fdatasync() instead of the default fsync().",
                         nullptr, nullptr, false);

2) Declare the underlying bool (srv_use_fdatasync) in srv0srv.h and 
define in srv0srv.cc:

/** Use fdatasync() instead of fsync(). */
bool srv_use_fdatasync = false;

3) In os0file.cc, static int os_file_fsync_posix(os_file_t file) add the fdatasync 
call if available:

    const auto ret = srv_use_fdatasync ? fdatasync(file) : fsync(file);
    const auto ret = fsync(file);