Run-time posix_fallocate failures

Buildfarm member "rover_firefly" has been failing like this since
269e780 went in:

creating template1 database in /export/home/postgres/HEAD/pgsql.47842/src/test/regress/./tmp_check/data/base/1 ...
FATAL: could not allocate space for file "pg_xlog/xlogtemp.60072" using posix_fallocate: Invalid argument
 

Apparently OmniOS has a version of posix_fallocate that doesn't actually
work, or at least doesn't work on the specific filesystem rover_firefly
is using.  The Single Unix Spec says that that is the correct error to
return if the filesystem doesn't provide support:
[EINVAL]The len argument is less than zero, or the offset argument is less than zero, or the underlying file system
doesnot support this operation.
 

I think you'd better rejigger that patch so that it falls through to the
old implementation if posix_fallocate() fails.
        regards, tom lane