xfs_fsr: file reads should be O_DIRECT

xfs_fsr: file reads should be O_DIRECT

When running xfs_fsr on a sparse filesystem image containing
approximately 8 million extents and 80GB of data, I noticed that the
page cache grew and consumed all the memory in the machine. It turns
out that xfs_fsr is using direct IO to write data, but buffered IO
to read data. Convert the read side to use direct IO to prevent page
cache blowouts.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>