xfs: don't try to use the filestream allocator for metadata allocations

xfs_bmap_btalloc_nullfb has two entirely different control flows when
using the filestream allocator vs the regular one, but it get the
conditionals wrong and ends up mixing the two for metadata allocations.
Fix this by adding a missing userdata check and slight refactoring.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>