ocfs2: fix wrong check in ocfs2_direct_IO_get_blocks

contig_blocks gotten from ocfs2_extent_map_get_blocks cannot be compared
with clusters_to_alloc. So convert it to clusters first.

Signed-off-by: Joseph Qi <joseph.qi@huawei.com>
Reviewed-by: Weiwei Wang <wangww631@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>