xfs: fold xfs_rtallocate_extent into xfs_bmap_rtalloc

There isn't really much left in xfs_rtallocate_extent now, fold it into
the only caller.

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>