[fix][auditbeat] Reset file offset when re-reading from the beginning

[marc-gr]

What does this PR do?

When a utmp file is read and its size is smaller than before (which can be caused by an inode reuse, for example), we have to reset its offset, also, otherwise it will ignore any new events in that file until the size is bigger than its previous value.

Why is it important?

We were missing events in some edge cases.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
  - [ ] I have made corresponding changes to the documentation
  - [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Logs

The logs of the bug:

2021-03-01T17:16:07.665+1000	WARN	[login]	login/utmp.go:190	Unexpectedly, the file /var/log/wtmp is smaller than before (new: 35712, old: 39552) - reading whole file.
2021-03-01T17:16:07.665+1000	DEBUG	[login]	login/utmp.go:204	Reading file /var/log/wtmp (utmpFile={Inode:14 Path:/var/log/wtmp Size:35712 Offset:39552 Type:0})
2021-03-01T17:16:07.665+1000	DEBUG	[login]	login/utmp.go:279	Saving UTMP file record ({Inode:14 Path:/var/log/wtmp Size:35712 Offset:39552 Type:0})

As you can see, it is reading from an invalid offset and saving it.

2021-03-01T17:16:17.666+1000	DEBUG	[login]	login/utmp.go:204	Reading file /var/log/wtmp (utmpFile={Inode:14 Path:/var/log/wtmp Size:36096 Offset:39552 Type:0})
2021-03-01T17:16:17.666+1000	DEBUG	[login]	login/utmp.go:279	Saving UTMP file record ({Inode:14 Path:/var/log/wtmp Size:36096 Offset:39552 Type:0})
2021-03-01T17:16:17.666+1000	DEBUG	[login]	login/login.go:130	0 new login records.

Subsequent calls still ignore new events even if the size has grown.

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

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Build stats

• Build Cause: Pull request #24414 updated

• Start Time: 2021-03-08T11:45:04.006+0000

• Duration: 38 min 31 sec

• Commit: 80e970c

Test stats 🧪

Test	Results
Failed	0
Passed	337
Skipped	145
Total	482

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Test stats 🧪

Test	Results
Failed	0
Passed	337
Skipped	145
Total	482

[adriansr]

LGTM