block: Move power management code into a new source file

Move the code for runtime power management from blk-core.c into the
new source file blk-pm.c. Move the corresponding declarations from
<linux/blkdev.h> into <linux/blk-pm.h>. For CONFIG_PM=n, leave out
the declarations of the functions that are not used in that mode.
This patch not only reduces the number of #ifdefs in the block layer
core code but also reduces the size of header file <linux/blkdev.h>
and hence should help to reduce the build time of the Linux kernel
if CONFIG_PM is not defined.

Signed-off-by: Bart Van Assche <[email protected]>
Reviewed-by: Ming Lei <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Cc: Jianchao Wang <[email protected]>
Cc: Hannes Reinecke <[email protected]>
Cc: Johannes Thumshirn <[email protected]>
Cc: Alan Stern <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

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