Prevent recursion into the IO path under low memory conditions by using
GFP_NOIO in place of GFP_KERNEL when allocating a new command with
tcmu_alloc_cmd() and user ring space with tcmu_get_empty_block().

Link: https://lore.kernel.org/r/20191108082901.417950-1-damien.lemoal@wdc.comReported-by: Masato Suzuki <masato.suzuki@wdc.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Acked-by: Mike Christie <mchristi@redhat.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>