ZTS: small fix for SEEK_DATA/SEEK_HOLE tests (#16413)

Some libc's like uClibc lag the proper definition of SEEK_DATA
and SEEK_HOLE. Since we have only two files in ZTS which use
these definitons, let's define them by hand:

#ifndef SEEK_DATA
#define SEEK_DATA 3
#endif
#ifndef SEEK_HOLE
#define SEEK_HOLE 4
#endif

There should be no failures, because:

• FreeBSD has support for SEEK_DATA/SEEK_HOLE since FreeBSD 8
• Linux has it since Linux 3.1
• the libc will submit the parameters unchanged to the kernel

Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>