xfs_export_operations.commit_metadata

This is the commit_metadata export operation for XFS.

- Takes one inode to be committed.

- Forces the log up to the lsn of the inode.

- Doesn't force the log if the inode doesn't have a pincount.

Signed-off-by: Ben Myers <bpm@sgi.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
[bfields@citi.umich.edu: trivial whitespace fix]
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>