Set the maximum ZVOL transfer size correctly

[dweeezil]

ZoL had been setting max_sectors to UINT_MAX, but until Linux 3.19, it
the kernel artifically capped it at 1024 (BLK_DEF_MAX_SECTORS).
This cap was removed in torvalds/linux@34b48db. This patch changes
it to DMU_MAX_ACCESS (in sectors) and also changes the ASSERT in
dmu_tx_hold_write() to allow the maximum transfer size.

[dweeezil]

I'm a little nervous about the ASSERT change in dmu_tx_hold_write() only because the "<" test has been in illumos for so long, however this does seem to work properly on zvols under light testing where the maximum transfer size is used.

[behlendorf]

@dweeezil The change to the ASSERT looks right to me, I'm a little surprised that's never come up before. We should get pretty decent test coverage on this in the buildbots so we'll see.

[kernelOfTruth]

https://lkml.org/lkml/2014/9/6/123

well it's unfortunate that all of the older kernels seem to be left in the dust with this since performance-enhancing changes aren't backported

perhaps Redhat, Suse, Oracle will backport this change (and related commits)

[behlendorf]

Merged as:

ded576e Set the maximum ZVOL transfer size correctly