xfs: refactor xfs_log_force_lsn

Use the the smallest possible loop as preable to find the correct iclog
buffer, and then use gotos for unwinding to straighten the code.

Also fix the top of function comment while we're at it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>