ocfs2: fix issue that ocfs2_setattr() does not deal with new_i_size==i_size

ocfs2: fix issue that ocfs2_setattr() does not deal with new_i_size==i_size

The issue scenario is as following:

1. fallocate a large disk space(eg.  30G) with FALLOC_FL_KEEP_SIZE for
   a file whose i_size and disk size are 512(or other size).  After
   executing fallocate, i_size file is still 512, and the disk size became
   to 30G+512；

2. ftruncate the file to new_i_size which equal to inode->i_size.
   After executing ftruncate, disk space does not changes.  In other
   words, i_size file is still 512, and disk size is 30G+512.  But we want
   disk size to be 512.  This does not meet our expectations.

In order to solve the issue above, we modified ocfs2_setattr().  If
attr->ia_size != i_size_read(inode), it calls ocfs2_truncate_file(), and
truncate disk space to attr->ia_size.

Signed-off-by: Younger Liu <younger.liu@huawei.com>
Cc: Jie Liu <jeff.liu@oracle.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Sunil Mushran <sunil.mushran@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>