Change zfs-kmod-devel install path

Change zfs-kmod-devel install path

Install the common zfs kernel development headers under
/usr/src/zfs-<version>/ rather than in a kernel specific
directory.  The kernel specific build products such as
zfs_config.h and Modules.symvers are left installed under
/usr/src/zfs-<version>/<kernel>.

This was done to be consistent with where dkms expects
kernel module source to be packaged.  It also allows for
a common zfs-kmod-devel package which includes the headers,
and per-kernel zfs-kmod-devel-<kernel> packages.

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