IB/core: Get rid of redundant verb ib_destroy_mr

This was added in a thought of uniting all mr allocation
and deallocation routines but the fact is we have a single
deallocation routine already, ib_dereg_mr.

And, move mlx5_ib_destroy_mr specific logic into mlx5_ib_dereg_mr
(includes only signature stuff for now).

And, fixup the only callers (iser/isert) accordingly.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>
diff --git a/drivers/infiniband/ulp/iser/iser_verbs.c b/drivers/infiniband/ulp/iser/iser_verbs.c
index 5c9f565..7122186 100644
--- a/drivers/infiniband/ulp/iser/iser_verbs.c
+++ b/drivers/infiniband/ulp/iser/iser_verbs.c
@@ -334,7 +334,7 @@
 {
 	ib_free_fast_reg_page_list(pi_ctx->prot_frpl);
 	ib_dereg_mr(pi_ctx->prot_mr);
-	ib_destroy_mr(pi_ctx->sig_mr);
+	ib_dereg_mr(pi_ctx->sig_mr);
 	kfree(pi_ctx);
 }