Fix nasa#484, will check the lseek() return value in linux_sysmon_upd…

[myc-yang]

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• I reviewed the Contributing Guide.
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Describe the contribution
Fixes #484, will suppress ignored return value warning by casting lseek() as (void)

Testing performed

Expected behavior changes

• None

System(s) tested on

• Hardware: PC
• OS: Ubuntu 18.04
• Versions: [e.g. cFE 6.6, OSAL 4.2, PSP 1.3 for mcp750, any related apps or tools]

Additional context
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Third party code
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Contributor Info - All information REQUIRED for consideration of pull request
Michael Yang NASA/GSFC

[jphickey]

This doesn't seem to add value. If the lseek failed for whatever reason, the later read will fail, and this will end up following the error paths from there.

While I agree that the error checking here could be better, I'm not sure this improves it. As this is linux-specific code, we should at least capture errno and print the message.

Also - as a side note - 0 is the only valid output from this call because it is SEEK_SET. Any nonzero value means it did not seek correctly.

[myc-yang]

Need to include the ERRNO value to the OS_printf() output.

We really should include the errno message here

[myc-yang]

@jphickey - If lseek() returns a -1, should I skip out of the while loop as well?