xfs: remove the xfs_alloc_arg argument to xfs_bmap_btalloc_accounting

xfs_bmap_btalloc_accounting only uses the len field from args, but that
has just been propagated to ap->length field by the 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>