xfs: make the rtalloc start hint a xfs_rtblock_t

0 is a valid start RT extent, and with pending changes it will become
both more common and non-unique.  Switch to pass a xfs_rtblock_t instead
so that we can use NULLRTBLOCK to determine if a hint was set or not.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>