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author Liu Bo <liubo2009@cn.fujitsu.com> 1340963928 -0600
committer Chris Mason <chris.mason@fusionio.com> 1343075277 -0400

Btrfs: use mnt_want_write_file instead of mnt_want_write

mnt_want_write_file is faster when file has been opened for write.

Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>
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