Fix failure-to-read-man-page in commit 899bd785c.

Fix failure-to-read-man-page in commit 899bd785c.

posix_fallocate() is not quite a drop-in replacement for fallocate(),
because it is defined to return the error code as its function result,
not in "errno".  I (tgl) missed this because RHEL6's version seems
to set errno as well.  That is not the case on more modern Linuxen,
though, as per buildfarm results.

Aside from fixing the return-convention confusion, remove the test
for ENOSYS; we expect that glibc will mask that for posix_fallocate,
though it does not for fallocate.  Keep the test for EINTR, because
POSIX specifies that as a possible result, and buildfarm results
suggest that it can happen in practice.

Back-patch to 9.4, like the previous commit.

Thomas Munro

Discussion: https://postgr.es/m/1002664500.12301802.1471008223422.JavaMail.yahoo@mail.yahoo.com