Fix function code in error report

This bug causes a lseek() failure to be reported as a "could not open"
failure in the error message, muddling bug reports.  I introduced this
copy-and-pasteo in commit 78e12201.

Noticed while reviewing code for bug report #15221, from lily liang.  In
version 10 the affected function is only used by multixact.c and
commit_ts, and only in corner-case circumstances, neither of which are
involved in the reported bug (a pg_subtrans failure.)

Author: Álvaro Herrera