Fix lock order inversion with zvol_open()

Fix lock order inversion with zvol_open()

zfsonlinux issue #3681 - lock order inversion between zvol_open() and
dsl_pool_sync()...zvol_rename_minors()

Remove trylock of spa_namespace_lock as it is no longer needed when
zvol minor operations are performed in a separate context with no
prior locking state; the spa_namespace_lock is no longer held
when bdev->bd_mutex or zfs_state_lock might be taken in the code
paths originating from the zvol minor operation callbacks.

Signed-off-by: Boris Protopopov <boris.protopopov@actifio.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3681