test_custom_stats: Fix compilation warning

Michael Paquier <michael@paquier.xyz>

Commit: 33980eaa6df0b1358ba2e70edbe9efd56dbd7fa6
Author: Michael Paquier <michael@paquier.xyz>
Date: 2025-12-15T01:34:18Z
test_custom_stats: Fix compilation warning

I have fat-fingered an error message related to an offset while
switching the code to use pgoff_t.  Let's switch to the same error
message used in the rest of the tree for similar failures with fseeko(),
instead.

Per buildfarm members running macos: longfin, sifaka and indri.

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