pgsql: Handle posix_fallocate() errors.

Handle posix_fallocate() errors.

On some platforms, posix_fallocate() is available but may still return
EINVAL if the underlying filesystem does not support it.  So, in case
of an error, fall through to the alternate implementation that just
writes zeros.

Per buildfarm failure and analysis by Tom Lane.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/5b571bb8c8d2bea610e01ae1ee7bc05adcfff528

Modified Files
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src/backend/access/transam/xlog.c |   30 ++++++++++++------------------
1 file changed, 12 insertions(+), 18 deletions(-)