Export symbols dsl_pool_config_{enter,exit}

These are needed by consumers (i.e. Lustre) who wish to use the
dsl_prop_register() interface to register callbacks when pool
properties of interest change. This interface requires that the
DSL pool configuration lock is held when called.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1762