Export dmu_buf_rele() symbol

While I'd like to remove the various pragmas in module/zfs/dbuf.c.
There are consumers such as Lustre which still depend on dmu_buf_*
versions of the symbols. Until all consumers can be converted to
use only the dbuf_* names leave this symbol exported.

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