xfs: also use xfs_bmap_btalloc_accounting for RT allocations

Make xfs_bmap_btalloc_accounting more generic by handling the RT quota
reservations and then also use it from xfs_bmap_rtalloc instead of
open coding the accounting logic there.

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>