ocfs2: Let ocfs2_xa_prepare_entry() do space checks.

ocfs2_xattr_set_in_bucket() doesn't need to do its own hacky space
checking.  Let's let ocfs2_xa_prepare_entry() (via ocfs2_xa_set()) do
the more accurate work.  Whenever it doesn't have space,
ocfs2_xattr_set_in_bucket() can try to get more space.

Signed-off-by: Joel Becker <joel.becker@oracle.com>