Mention the bug related to Linux, xfs and rsync on /manual/administration/backups/ page

In relation of the bug https://jira.mongodb.org/browse/SERVER-8518 I'm proposing a following note to the page http://docs.mongodb.org/manual/administration/backups/

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Nearly all Linux kernel versions up to 2.6.32 suffers a bug https://bugzilla.kernel.org/show_bug.cgi?id=2645 which in some cases delays the mtime (modification timestamp) update for the mongodb data files when using the XFS filesystem. Also other filesystems might be affected in older Linux versions. This delay might cause rsync to ignore some data files if rsync is used to make backups.

You can verify if you might be affected by this bug by compiling and running this test script https://bugzilla.kernel.org/attachment.cgi?id=14430

Always remember to test and practice backup recovery, so you can be sure you have a working backup method.