Add module versioning

Use the standard Linux MODULE_VERSION macro to expose the installed
spl and splat module versions.  This will also automatically add a
checksum of the .c files and headers in "srcversion".  See:

  /sys/module/spl/version
  /sys/module/spl/srcversion
  /sys/module/splat/version
  /sys/module/splat/srcversion

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes zfsonlinux/zfs#1923

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