xfs: remove IO submission from xfs_reclaim_inode()

We no longer need to issue IO from shrinker based inode reclaim to
prevent spurious OOM killer invocation. This leaves only the global
filesystem management operations such as unmount needing to
writeback dirty inodes and reclaim them.

Instead of using the reclaim pass to write dirty inodes before
reclaiming them, use the AIL to push all the dirty inodes before we
try to reclaim them. This allows us to remove all the conditional
SYNC_WAIT locking and the writeback code from xfs_reclaim_inode()
and greatly simplify the checks we need to do to reclaim an inode.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>