xfs: return -ENOSPC from xfs_rtallocate_*

Just return -ENOSPC instead of returning 0 and setting the return rt
extent number to NULLRTEXTNO.  This is turn removes all users of
NULLRTEXTNO, so remove that as well.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

fs/xfs/libxfs/xfs_types.h

@@ -51,7 +51,6 @@ typedef void *		xfs_failaddr_t;
 #define	NULLRFSBLOCK	((xfs_rfsblock_t)-1)
 #define	NULLRTBLOCK	((xfs_rtblock_t)-1)
 #define	NULLFILEOFF	((xfs_fileoff_t)-1)
-#define	NULLRTEXTNO	((xfs_rtxnum_t)-1)
 
 #define	NULLAGBLOCK	((xfs_agblock_t)-1)
 #define	NULLAGNUMBER	((xfs_agnumber_t)-1)