RDMA/irdma: Fix double free related to rereg_user_mr

If IB_MR_REREG_TRANS is set during rereg_user_mr, the
umem will be released and a new one will be allocated
in irdma_rereg_mr_trans. If any step of irdma_rereg_mr_trans
fails after the new umem is allocated, it releases the umem,
but does not set iwmr->region to NULL. The problem is that
this failure is propagated to the user, who will then call
ibv_dereg_mr (as they should). Then, the dereg_mr path will
see a non-NULL umem and attempt to call ib_umem_release again.

Fix this by setting iwmr->region to NULL after ib_umem_release.

Fixed: 5ac388db27c4 ("RDMA/irdma: Add support to re-register a memory region")
Signed-off-by: Jacob Moroni <jmoroni@google.com>
Link: https://patch.msgid.link/20260227152743.1183388-1-jmoroni@google.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>