xfs: don't hold onto reserved blocks on remount,ro

xfs: don't hold onto reserved blocks on remount,ro

If we hold onto reserved blocks when doing a remount,ro we end
up writing the blocks used count to disk that includes the reserved
blocks. Reserved blocks are not actually used, so this results in
the values in the superblock being incorrect.

Hence if we run xfs_check or xfs_repair -n while the filesystem is
mounted remount,ro we end up with an inconsistent filesystem being
reported. Also, running xfs_copy on the remount,ro filesystem will
result in an inconsistent image being generated.

To fix this, unreserve the blocks when doing the remount,ro, and
reserved them again on remount,rw. This way a remount,ro filesystem
will appear consistent on disk to all utilities.

Signed-off-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>