xfs: add the xlog_grant_head structure

Add a new data structure to allow sharing code between the log grant and
regrant code.

Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

diff --cc fs/xfs/xfs_log.c
Simple merge

diff --cc fs/xfs/xfs_log_priv.h
Simple merge

diff --cc fs/xfs/xfs_log_recover.c
Simple merge

diff --cc fs/xfs/xfs_trace.h
Simple merge