ocfs2: take inode lock when get clusters

We need take inode lock when calling ocfs2_get_clusters.
And use GFP_NOFS instead of GFP_KERNEL.

Signed-off-by: Joseph Qi <joseph.qi@huawei.com>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>