xfs: fix backwards logic in xfs_bmap_alloc_account

We're only allocating from the realtime device if the inode is marked
for realtime and we're /not/ allocating into the attr fork.

Fixes: 58643460 ("xfs: also use xfs_bmap_btalloc_accounting for RT allocations")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>