Move posix_fadvise to xb_file_set_nocache

[Raghavendra D Prabhu]

In xtrabackup code I noticed posix_fadvise being used in O_DIRECT context as well.

Mainly,
 posix_fadvise(src_file, 0, 0, POSIX_FADV_SEQUENTIAL);
 posix_fadvise(src_file, 0, 0, POSIX_FADV_DONTNEED);

POSIX_FADV_SEQUENTIAL is helpful only while doing non direct-io.

Regarding POSIX_FADV_DONTNEED, it doesn't harm but it is redundant since filesystem code will invalidate the cached pages in any case, before doing the direct-io. (it should to maintain consistency so all filesystems do that).

They can be moved to xb_file_set_nocache to the 'else' condition of

 if (srv_unix_file_flush_method == SRV_UNIX_O_DIRECT) {
  os_file_set_nocache(fd, file_name, operation_name);
 }

[Alexey Kopytov (akopytov)]

See also bug #1096828.

[Alexey Kopytov (akopytov)]

I think we discussed elsewhere that this report assumes specific implementation and there is actually no negative impact if posix_fadvise() hints are used correctly.

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PXB-85