Cleanup export list

This commit makes a couple of internal functions static so they're not
exported from the module.

Before:
```
$ nm srb.ko | grep ' T ' | grep -v ' srb_'
0000000000000000 T cleanup_module
0000000000003f60 T human_to_bytes
0000000000000000 T init_module
0000000000007a80 T jsmn_init
0000000000007560 T jsmn_parse
0000000000003cb0 T parse_params
0000000000000910 T req_code_to_str
0000000000000970 T req_flags_to_str
00000000000005e0 T _srb_volume_dump
```

After:
```
$ nm srb.ko | grep ' T ' | grep -v ' srb_'
0000000000000000 T cleanup_module
0000000000000000 T init_module
0000000000007a90 T jsmn_init
0000000000007570 T jsmn_parse
```
Don't set `__KERNEL__` and `MODULE` defines
Make `srb_xfer_scl` static
Remove duplicate include

Found by `checkincludes.pl`.
Handle `krealloc` NULL-result

Found by `checkpatch.pl`:

```
WARNING: Reusing the krealloc arg is almost always a bug
+               json_tokens = krealloc(json_tokens,
```

See: https://lkml.org/lkml/2013/3/14/405
Use `kmalloc_array`

Suggested by `checkpatch.pl`:

```
WARNING: Prefer kmalloc_array over kmalloc with multiply
+       dev->thread_cdmi_desc = kmalloc(sizeof(struct srb_cdmi_desc_s *) * thread_pool_size, GFP_KERNEL);
```

```
WARNING: Prefer kmalloc_array over kmalloc with multiply
+       dev->thread = kmalloc(sizeof(struct task_struct *) * thread_pool_size, GFP_KERNEL);
```
Fix HTTP status code

Typo, '40 ' instead of '406'.

Found by accident through `checkpatch.pl`:

```
ERROR: space prohibited before that ':' (ctx:WxW)
+               case 400: case 401: case 402: case 403: case 404: case 405: case 40 : case 407: case 408: case 409:
```
Use `kstrtou64` instead of `sscanf`

Suggested by `checkpatch.pl`:

```
WARNING: Prefer kstrto<type> to single variable sscanf
+       ret = sscanf(&buff[ipos+keylen+span], "%lu", (unsigned long *)value);
```
Merge pull request #64 from scality/cleanups

Some cleanups