IB/mlx5: Fix function updating xlt emergency path

In memory shortage path we fall back to use spare buffer.
mlx5_ib_update_xlt() called from ib_uverbs_reg_mr when ibmr.ucontext
not initialized yet.

Scenario how to test it:
1. trigger memory exhaustion so __get_free_pages(GFP_KERNEL, 4) will fail
2. register MR
3. there should be no kernel oops

Fixes: 7d0cc6ed ('IB/mlx5: Add MR cache for large UMR regions')
Signed-off-by: Artemy Kovalyov <artemyko@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>