RDMA/rxe: Fix a couple IS_ERR() vs NULL bugs

RDMA/rxe: Fix a couple IS_ERR() vs NULL bugs

The lookup_mr() function returns NULL on error.  It never returns error
pointers.

Fixes: 9284bc34c773 ("RDMA/rxe: Enable asynchronous prefetch for ODP MRs")
Fixes: 3576b0df1588 ("RDMA/rxe: Implement synchronous prefetch for ODP MRs")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://patch.msgid.link/685c1430.050a0220.18b0ef.da83@mx.google.com
Reviewed-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Signed-off-by: Leon Romanovsky <leon@kernel.org>