Fixed a use-after-free bug in zfs_zget().

Fixed a bug where zfs_zget could access a stale znode pointer when
the inode had already been removed from the inode cache via iput ->
iput_final -> ... -> zfs_zinactive but the corresponding SA handle
was still alive.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #180