Create /proc/sys/kernel/spl/gitrev with git hash

Create /proc/sys/kernel/spl/gitrev with git hash

The existing mechanisms for determining what code is running in the
kernel do not always correctly report the git hash.  The versions
reported there do not reflect changes made since `configure` was run
(i.e. incremental builds do not update the version) and they are
misleading if git tags are not set up properly.  This applies to
`modinfo zfs`, `dmesg`, and `/sys/module/zfs/version`.

There are complicated requirements on how the existing version is
generated.  Therefore we are leaving that alone, and adding a new
mechanism to record and retrieve the git hash:
`cat /proc/sys/kernel/spl/gitrev`

The gitrev is re-generated at compile time, when running `make`
(including for incremental builds).  The value is the output of `git
describe` (or "unknown" if not in a git repo or there are uncommitted
changes).

We're also removing /proc/sys/kernel/spl/version, which was never very
useful.

Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Tim Chase <tim@chase2k.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #7931
Closes #7965