Btrfs: pass root object into btrfs_ioctl_{start, wait}_sync()

Since we have gotten the root in the caller, just pass it into
btrfs_ioctl_{start, wait}_sync() directly.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
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