Export correct symbols for Lustre Direct I/O

Originally the Lustre ZFS OSD code was going to use zfs_uio_t structs
for supporting Direct I/O with ZFS. However, this has changed to using
abd_t structs instead. This exports the proper symbols that will be used
by the Lustre ZFS OSD code.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Brian Atkinson <batkinson@lanl.gov>
Closes #17256

module/zfs/abd.c

@@ -1210,3 +1210,5 @@ abd_raidz_rec_iterate(abd_t **cabds, abd_t **tabds,
 	}
 	abd_exit_critical(flags);
 }
+
+EXPORT_SYMBOL(abd_free);