debian: Use dh_installmodules instead of calling "depmod" wrongly.

debian: Use dh_installmodules instead of calling "depmod" wrongly.

Until now, the postinst for kernel modules built by the Debian packaging
has simply run "depmod -a", which is wrong, since this command rebuilds
the dependencies for the *running* kernel, which is not necessarily the
kernel for which modules are being installed.

The dh_installmodules script automatically adds the correct invocation of
depmod to the postinst script, so this commit switches to using that
instead.

This commit moves the kernel modules from /lib/modules/$KVERS into the
"kernel" subdirectory of that directory because dh_installmodules does not
support modules that are directly in the $KVERS directory.

CC: Sajjad Lateef <slateef@nicira.com>