Rename macro ZFS_MINOR due to Lustre conflict

Macro ZFS_MINOR, introduced in commit a6cc9756 to record the chosen
static minor number for /dev/zfs, conflicts with an existing macro
in Lustre. The lustre macro (along with _MAJOR, _PATCH, _FIX) is
used to record the zfsonlinux version Lustre is being built against.

Since the Lustre macro came first, and is used in past versions of
lustre at least going back to 2.10, it makes sense to rename the
macro in ZFS instead of doing so in Lustre which would require
backporting the patch.

Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #8195