RDMA/rxe: Replace exists by rxe in rxe.c

RDMA/rxe: Replace exists by rxe in rxe.c

'exists' looks like a boolean. This patch replaces it by the
normal name used for the rxe device, 'rxe', which should be a
little less confusing. The second rxe_dbg() message is
incorrect since rxe is known to be NULL and this will cause a
seg fault if this message were ever sent. Replace it by pr_debug
for the moment.

Fixes: c6aba5ea0055 ("RDMA/rxe: Replace pr_xxx by rxe_dbg_xxx in rxe.c")
Link: https://lore.kernel.org/r/20230303221623.8053-2-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>