ZTS: limit mmapwrite file size

mmapwrite spawns several threads, all of which perform writes on a file
for the purpose of testing the behavior of mmap(2)-ed files. One
thread performs an mmap and a write to the beginning of that region,
while the others perform regular writes after lseek(2)-ing the end of
the file.

Because these regular writes are set in a while (1) loop, they will
write an unbounded amount of data to disk. The mmap_write_001_pos test
script SIGKILLs them after 30 seconds, but on fast testbeds, this may
be enough time to exhaust the available space in the filesystem,
leading to spurious test failures.

Instead, limit the total file size by checking that the lseek return
value is no greater than 250 * 1024*1024 bytes, which is less than the
default minimum vdev size defined in includes/default.cfg .

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Antonio Russo <aerusso@aerusso.net>
Closes #14277
Closes #14345