xfs: fold xfs_bmap_alloc_userdata into xfs_bmapi_allocate

Userdata and metadata allocations end up in the same allocation helpers.
Remove the separate xfs_bmap_alloc_userdata function to make this more
clear.

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