xfs: cleanup xfs_log_unmount_write

Move the code for verifying the iclog state on a clean unmount into a
helper, and instead of checking the iclog state just rely on the shutdown
check as they are equivalent.  Also remove the ifdef DEBUG as the
compiler is smart enough to eliminate the dead code for non-debug builds.

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