IB/umem: Indicate that process is being terminated

When process is killed while pagefault operation still in progress -
function will fail. In this specific case we don't want any warnings in
dmesg to avoid log analyzers false alerts. So we need distinct error
code for this case.

Signed-off-by: Artemy Kovalyov <artemyko@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>