net/mlx5: Fix failing fw tracer allocation on s390

On s390 FORCE_MAX_ZONEORDER is 9 instead of 11, thus a larger kzalloc()
allocation as done for the firmware tracer will always fail.

Looking at mlx5_fw_tracer_save_trace(), it is actually the driver itself
that copies the debug data into the trace array and there is no need for
the allocation to be contiguous in physical memory. We can therefor use
kvzalloc() instead of kzalloc() and get rid of the large contiguous
allcoation.

Fixes: f53aaa31

 ("net/mlx5: FW tracer, implement tracer logic")
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>