ocfs2: Move 'wanted' into parens of ocfs2_resmap_resv_bits.

The first time I read the function ocfs2_resmap_resv_bits, I consider
about what 'wanted' will be used and consider about the comments.
Then I find it is only used if the reservation is empty. ;)

So we'd better move it to the parens so that it make the code more
readable, what's more, ocfs2_resmap_resv_bits is used so frequently
and we should save some cpus.

Acked-by: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>