ocfs2: Take ocfs2_xattr_bucket structures off of the stack.

The ocfs2_xattr_bucket structure is a nice abstraction, but it is a bit
large to have on the stack.  Just like ocfs2_path, let's allocate it
with a ocfs2_xattr_bucket_new() function.

We can now store the inode on the bucket, cleaning up all the other
bucket functions.  While we're here, we catch another place or two that
wasn't using ocfs2_read_xattr_bucket().

Updates:
- No longer allocating xis.bucket, as it will never be used.
Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>