posix_fadvise versus old kernels

I've been digging into why buildfarm member thrush has been dumping core
consistently during the regression tests since the posix_fadvise patch
went in.  I've confirmed that posix_fadvise() itself will SIGSEGV in a
standalone test program, and found that this happens only if
_FILE_OFFSET_BITS=64 ... which is our default configuration on Linux.

Some googling turned up the following 
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=313219
which basically says that posix_fadvise64 + 2.4 kernel + older glibc
= crash.  It sounds like the 2.4 kernel hasn't got this call but glibc
thought it did, up till about a year ago.

While we could possibly come up with a suitable configure test to
determine whether posix_fadvise is actually safe to use on a given
system, I think we should seriously consider just reverting the patch.
As far as I saw, zero evidence was given that it actually does anything
measurable.  Without a benchmark to prove that it's worth spending more
time on, I'm disinclined to trouble over it.
        regards, tom lane