MFV r316928: 7256 low probability race in zfs_get_data

MFV r316928: 7256 low probability race in zfs_get_data

illumos/illumos-gate@0c94e1af6784c69a1dea25e0e35dd13b2b91e2e5
https://github.com/illumos/illumos-gate/commit/0c94e1af6784c69a1dea25e0e35dd13b2b91e2e5

https://www.illumos.org/issues/7256
                         error = dmu_sync(zio, lr->lr_common.lrc_txg,
                              zfs_get_done, zgd);
                         ASSERT(error || lr->lr_length <= zp->z_blksz);
  It's possible, although extremely rare, that the zfs_get_done() callback is
  executed before dmu_sync() returns.
  In that case the znode's range lock is dropped and the znode is unreferenced.
  Thus, the assertion can access some invalid or wrong data via the zp pointer.
  size variable caches the correct value of z_blksz and can be safely used here.

Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Andriy Gapon <andriy.gapon@clusterhq.com>

MFC after:	1 week