[XFS] Allow 64 bit machines to avoid the AIL lock during flushes

When copying lsn's from the log item to the inode or dquot flush lsn, we
currently grab the AIL lock. We do this because the LSN is a 64 bit
quantity and it needs to be read atomically. The lock is used to guarantee
atomicity for 32 bit platforms.

Make the LSN copying a small function, and make the function used
conditional on BITS_PER_LONG so that 64 bit machines don't need to take
the AIL lock in these places.

SGI-PV: 988143

SGI-Modid: xfs-linux-melb:xfs-kern:32349a

Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>