devlink: don't allocate attrs on the stack

Number of devlink attributes has grown over 128, causing the
following warning:

../net/core/devlink.c: In function ‘devlink_nl_cmd_region_read_dumpit’:
../net/core/devlink.c:3740:1: warning: the frame size of 1064 bytes is larger than 1024 bytes [-Wframe-larger-than=]
 }
  ^

Since the number of attributes is only going to grow allocate
the array dynamically.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>