Modern versions of dkms cleanup the build directory after installing.
This resulted in 'dkms uninstall' never running because the check
added by commit 866c162 which verifies the existance of the
zfs.release build product would never be true.
This patch resolves the issue by updating the conditional to check
in the explicitly installed zfs_config.h file for the version.
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#3862
3ef005c674