Revert "Revert "Revert "Fix unlink/xattr deadlock"""

This reverts commit 7973e46 which brings the basic flow of the
code back in line with the other ZFS implementations.  This
was possible due to the following related changes.

e89260a Directory xattr znodes hold a reference on their parent
6f9548c Fix deadlock in zfs_zget()
ca043ca Add zfs_iput_async() interface
dd2a794 Avoid 128K kmem allocations in mzap_upgrade()

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #457
Issue #2058
Issue #2128
Issue #2240