IB/srp: Fix a memory descriptor leak in an error path

If an error occurs after srp_fr_pool_get() succeeded and before the
descriptor is stored in srp_map_state (*state->fr.next++ = desc)
then srp_unmap_data() won't free the newly allocated memory
descriptor. Hence free the descriptor explicitly.

Fixes: f7f7aab1 ("IB/srp: Convert to new registration API")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Tested-by: Laurence Oberman <loberman@redhat.com>
Cc: Sagi Grimberg <sai@grimberg.me>
Cc: Christoph Hellwig <hch@lst.de>
Cc: <stable@vger.kernel.org> # v4.4+
Signed-off-by: Doug Ledford <dledford@redhat.com>