Export symbol dmu_tx_mark_netfree()

Export symbol dmu_tx_mark_netfree()

This symbol is needed by Lustre for the same reason it was needed
by the ZPL.  It should have been exported when the original patch
was merged.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6660