Refactor zvol locking to minimise diff with upstream

Use #define zfsdev_state_lock spa_namespace_lock instead of replacing all
zfsdev_state_lock with spa_namespace_lock to minimise changes from upstream.

Differential Revision:	D1302
MFC after:	1 month
X-MFC-With	r276063
Sponsored by:	Multiplay


git-svn-id: svn+ssh://svn.freebsd.org/base/head@276066 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f