xfs: reinstate RT support in xfs_bmapi_reserve_delalloc

Allocate data blocks for RT inodes using xfs_dec_frextents.  While at
it optimize the data device case by doing only a single xfs_dec_fdblocks
call for the extent itself and the indirect blocks.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>