Linux 5.3 compat: Makefile subdir-m no longer supported

Uses obj-m instead, due to kernel changes.

See LKML: Masahiro Yamada, Tue, 6 Aug 2019 19:03:23 +0900

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Dominic Pearson <dsp@technoanimal.net>
Closes #9169