xfs: refactor xfs_bmapi_allocate

Avoid duplicate userdata and data fork checks by restructuring the code
so we only have a helper for userdata allocations that combines these
checks in a straight foward way.  That also helps to obsoletes the
comments explaining what the code does as it is now clearly obvious.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>