xfs: call xfs_bmap_exact_minlen_extent_alloc from xfs_bmap_btalloc

xfs_bmap_exact_minlen_extent_alloc duplicates the args setup in
xfs_bmap_btalloc.  Switch to call it from xfs_bmap_btalloc after
doing the basic setup.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>