staging: lustre: Use 'kvfree()' for memory allocated by 'kvzalloc()'

'buf' is allocated with 'kvzalloc()'. 'kvfree()' must be used to free it.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Fixes: 11c647ca ("staging: lustre: obdclass: variable llog chunk size")
Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>