Remove FreeBSD's local copy of the dmu_buf_hold_array() function

Make the main dmu_buf_hold_array() function non-static.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Pawel Jakub Dawidek <pawel@dawidek.net>
Closes #12628