Export symbols for the VFS API

Export all symbols already marked extern in the zfs_vfsops.h
header.  Several non-static symbols have also been added to
the header and exportewd.  This allows external modules to
more easily create and manipulate properly created ZFS
filesystem type datasets.

Rename zfsvfs_teardown() to zfs_sb_teardown and export it.
This is done simply for consistency with the rest of the code
base.  All other zfsvfs_* functions have already been renamed.

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