The regular read req asks for
mapping the block with mirror_num == 0, in this case the
source disk is avoided whenever possible. The repair code
selects the mirror_num explicitly (mirror_num != 0), this
case is not changed by this commit.

Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
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