Add get_gsh_export_ref_safe to safely get an export reference.

The caller MUST hold a lock that prevents put_gsh_export of the last
reference from proceeding. It could also be a lock that prevents
pseudo_unmount_export or release_export_root from completing. This
assures that the pointer is sufficiently valid to examine if it is
on the export list.

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>
MORE Add refcount to state_t

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>
TEMP

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>