[writeboost] Use dm_kcopyd_zero to zero out a region

Memory allocation by vmalloc may fail if kernel outside Writeboost
allocated lot of memory before Writeboost does. This because the total
amount of vmalloc-ed space is limited. So, generally, using vmalloc is
not preferable in kernel.

This patch uses dm_kcopyd_zero routine to zero out region in device.
Using this routine instead can remove dependency to vmalloc to get
zero-filled buffer.

do_zeroing_region() wrapper is defined to do this work synchonously and two
other routines (zeroing_full_superblock and do_clear_plog_dev_t1) use it
now. As a result, no vmalloc() can be seen in Writeboost.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>

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v2: Rebased to the current thin-dev