mlx4: allocate just enough pages instead of always 4 pages

The driver uses a 2-order allocation, which is too much on architectures
like ppc64, which has a 64KiB page. This particular allocation is used
for large packet fragments that may have a size of 512, 1024, 4096 or
fill the whole allocation. So, a minimum size of 16384 is good enough
and will be the same size that is used in architectures of 4KiB sized
pages.

This will avoid allocation failures that we see when the system is under
stress, but still has plenty of memory, like the one below.

This will also allow us to set the interface MTU to higher values like
9000, which was not possible on ppc64 without this patch.

Node 1 DMA: 737*64kB 37*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB 0*8192kB 0*16384kB = 51904kB
83137 total pagecache pages
0 pages in swap cache
Swap cache stats: add 0, delete 0, find 0/0
Free swap  = 10420096kB
Total swap = 10420096kB
107776 pages RAM
1184 pages reserved
147343 pages shared
28152 pages non-shared
netstat: page allocation failure. order:2, mode:0x4020
Call Trace:
[c0000001a4fa3770] [c000000000012f04] .show_stack+0x74/0x1c0 (unreliable)
[c0000001a4fa3820] [c00000000016af38] .__alloc_pages_nodemask+0x618/0x930
[c0000001a4fa39a0] [c0000000001a71a0] .alloc_pages_current+0xb0/0x170
[c0000001a4fa3a40] [d00000000dcc3e00] .mlx4_en_alloc_frag+0x200/0x240 [mlx4_en]
[c0000001a4fa3b10] [d00000000dcc3f8c] .mlx4_en_complete_rx_desc+0x14c/0x250 [mlx4_en]
[c0000001a4fa3be0] [d00000000dcc4eec] .mlx4_en_process_rx_cq+0x62c/0x850 [mlx4_en]
[c0000001a4fa3d20] [d00000000dcc5150] .mlx4_en_poll_rx_cq+0x40/0x90 [mlx4_en]
[c0000001a4fa3dc0] [c0000000004e2bb8] .net_rx_action+0x178/0x450
[c0000001a4fa3eb0] [c00000000009c9b8] .__do_softirq+0x118/0x290
[c0000001a4fa3f90] [c000000000031df8] .call_do_softirq+0x14/0x24
[c000000184c3b520] [c00000000000e700] .do_softirq+0xf0/0x110
[c000000184c3b5c0] [c00000000009c6d4] .irq_exit+0xb4/0xc0
[c000000184c3b640] [c00000000000e964] .do_IRQ+0x144/0x230

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>
Signed-off-by: Kleber Sacilotto de Souza <klebers@linux.vnet.ibm.com>
Tested-by: Kleber Sacilotto de Souza <klebers@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>