Fix depmod warning

Fix depmod warning

The depmod utility from module-init-tools 3.12-pre3 generates a
warning when the -e option is used without -E or -F.  This was
observed under OpenSuse 11.4.  To resolve the issue when the
exact System.map-* for your kernel cannot be found fallback to
a generic safe '/sbin/depmod -a'.

  WARNING: -e needs -E or -F

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