Implement SEEK_HOLE/SEEK_DATA for ext2fs.

Merged from r236044 on UFS.

MFC after:	3 days
Remove all the NOPs after SYNC. They aren't needed.

They originated in the original Octeon port. They weren't present, as
far as I can tell, on the projects/mips branch until after this
point. They were in the original Octeon port in code picked up from
the vendor, who I've been able to find out trolling old email put them
there to get around an SMP problem that most likely was fixed in other
ways.

NetBSD and Linux don't have these, except for some specific uses of
SYNC on the alchemy parts (which we don't support, but even if we did
it is only a specific case and would be specifically coded
anyway). This is true of the current Linux code, as well as one old
version I polled.

I looked back at the old R12000, R8000, R6000, R4000, R4400 errata
that I have, and could find no mention of SYNC needing NOPs for
silicon bugs (although plenty of other cases where NOPs and other
contortions were needed).

An Google search turned up no old mailing list discussions on this on
Linux, NetBSD or FreeBSD (except the disussion that kicked off these
studies).

I've test booted this on my Octeon Plus eval board and survived a
buildworld. Adrian Chadd reports that this patch has no ill effects on
the Ahteros platforms he tested it on.

I conclude it is safe to just remove the NOPs. But added
__MIPS_PLATFORM_SYNC_NOPS as a failsafe in case we find some platform
where these are, in fact, required.

Reviewed by:	adrian@