Remove extra newline from spa_set_allocator().

zfs_dbgmsg() does not need newline at the end of the message.

While there, slightly update/sync FreeBSD __dprintf().

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16536