Various zimport.sh fixes

1) $SPLSRC and $SRCDIR should be changed to $SRC_DIR.  These are
   vestiges of an earlier version of the script and were missed when
   it was updated.  Additionally ensure the directory is created.

2) The 'fail' function should take an integer argument for the
   error code to return.  Otherwise 0 (success) will be mistakenly
   returned and errors will we incorrectly suppressed.  The error
   code should be meaningful enough to determine where the script
   failed.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>