dsl_dir_tempreserve_impl: remove unused deferred variable

The following commit moved the users of deferred into function
dsl_pool_unreserved_space:

    commit d2734cce68cf740e015312314415f9034c67851c
    Author: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
    Date:   Fri Dec 16 14:11:29 2016 -0800

        OpenZFS 9166 - zfs storage pool checkpoint

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <freqlabs@FreeBSD.org>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Christian Schwarz <christian.schwarz@nutanix.com>
Closes #13056