vfs: move zfs_init to vfs switch table

vfs_init goes through the table automatically calling initialization
functions from each fs there installed, so zfs_init will be properly
called during the vfs initialization process.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>