compat: fix dma_common_get_sgtable() backport placements

compat: fix dma_common_get_sgtable() backport placements

We added dma_common_get_sgtable() assuming it should only
be available to older kernels as of 3.6 but it was only
up to 3.8 where it was detangled from depending on
CONFIG_HAVE_GENERIC_DMA_COHERENT. Its a mess to express
this on compat files so just word it via kconfig and throw this
onto its own files given that there may be other helpers
we may need to backport later with the same circumstances.

One last hack was required for 3.3-3.6 and that is due
to header changes between kernels.

$ time ckmake --check-exports --allyesconfig

1   2.6.24              [  OK  ]
2   2.6.25              [  OK  ]
3   2.6.26              [  OK  ]
4   2.6.27              [  OK  ]
5   2.6.28              [  OK  ]
6   2.6.29              [  OK  ]
7   2.6.30              [  OK  ]
8   2.6.31              [  OK  ]
9   2.6.32              [  OK  ]
10  2.6.33              [  OK  ]
11  2.6.34              [  OK  ]
12  2.6.35              [  OK  ]
13  2.6.36              [  OK  ]
14  2.6.37              [  OK  ]
15  2.6.38              [  OK  ]
16  2.6.39              [  OK  ]
17  3.0.65              [  OK  ]
18  3.1.10              [  OK  ]
19  3.2.38              [  OK  ]
20  3.3.8               [  LINK  ]
21  3.4.32              [  LINK  ]
22  3.5.7               [  LINK  ]
23  3.6.11              [  OK  ]
24  3.7.9               [  OK  ]
25  3.8.0               [  OK  ]
26  3.9-rc1             [  OK  ]

real    30m3.120s
user    795m40.684s
sys     123m29.995s

Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>